This is a tough question. There is one more factor (like you need more) to consider here, and that is that kindergarten red shirting is very trendy right now. She is likely to find that she is in a first grade class made up of more 7 year olds than 6 year olds, and some may be nearly 8. You may want to find out how often this is happeneing at the school she will attend. This has a significant impact on socialization and peer relations. I expect that the curiculum based standards are in for a huge adjustment very soon. It certainly makes the normative data (based on age) more difficult to apply to your child.
The other factor to consider is how giftedness is identified. There is a large element of the early learners for whom the quick learning will slow down, and they will find themselves in the big part of the bell curve by the time they start 4th grade. This may have had some bearing on what happened to you in 5th grade, especially since you were bumped up two grade levels early on.
Through third grade, children are primarily learning to read and write, and suddenly, when the inverse is true, and they are reading and writing to learn, the expanded demands of conceptual learning can overwhelm some who do very well at learing the mechanics of reading and writing, and the learning speed slows down to a more average pace. For some that had difficulty with mechanics, conceptual learning is more fluid, which is why early intervention is so crucial so that these otherwise gifted children can move on to conceptual material with their grade peers, or even move on to a gifted program based on their conceptual ablities.
Math is different, and you will notice that children are identified for math programs seperately, because conceptual skills are part of the early learning process along with memorization skills for math facts. The concepts become progressively more complex and cumulative, but some children who memorize math facts readily will find math concepts (fluid reasoning skills) more difficult and vice verca, some children had great difficulty with memorization and have little difficulty understanding math concepts because they are two different skill sets. Memorization and memory retrieval skills continue to be important for math.
It is important for you to know which of these skills are strong for your daughter so that you can know the best placement for her, whether you expect for her quick pace to continue or even out, and the weigh her social ease and maturity levels carefully with the age of actual students in the class, not just the age you would normally expect to find in a first grade classroom.
If you can get a copy of the testing that the school did on your daughter, you might see what kind they did. If it was curriculum based (meaning, she was measured compaired to the content of the curiculum, or how much she knows about reading, writing, spelling, math...) Then you might want to invest in an evaluation that will tell you how she processes information and how strong her skills are in more psychoeducational terms. This will include an IQ test that measures many differnet processes, and will give you standard scores that relate to her age. The problem here, is always going to be that you won't have her processing skills compared to the kids in her expected grade, which now, more than ever, will include children who are as much as three years appart from the youngest to the oldest.
If you have standard tests, like an IQ test from the assessment the school did, there is an article at www.wrightslaw.com called "Understanding Tests and Measurments for Parents and Advocates." While written for parents with children who have special needs, it will give you the kind of understanding that will be helpful to you to make educational decisions for your daughter too.
As an advocate for children with disablities, my advice to parents is always to send their children to school on time, but that is usually in response to the question "should I hold my child back?" The answer to that question is a resounding 'NO!" because the data is clear that holding children back is a very bad educational strategy for children with special needs, and has very poor measurable educational outcomes assoicated with it. Children who are older than the grade expectation are at very high risk for reading failure. Children who enter school on time are at an advantage academically becasuse they get more years of instruction while thier brains are still flexible and elastic and can learn language and verbal skills most easliy. That window closes around age 9, so kids who need reading intervention actually get a full year more of targeted reading instruction before that elesticity is gone, and children who are older don't benefit from that extra year. Even though children have been entering the system later, intervention is still based on grade, not age. Children with the misfortune to need reading intervention, enter school late for age, and be enrolled in an unresponsive school system that does not identify properly, may not get the instruction untill it is way too late for them to the most productive and easier gains; they also end up learning to read and write while thier grade peers have moved on to the process of reading and writing to learn. It is an educational disaster.
The question is for you is, is that educational advantage extended to your daughter by entering the system earlier? That is how I would approach the situation. I know the answer if your child is at the left side of the bell curve, but being on the right is more difficult to answer.
I can share an anecdotal answer, which is much less reliable than data, but you may find interesting. One of my neigbors has a child who is very gifted; Dougie Howser gifted. His parents sent him to kindergarten on time. He plays with kids his own age, and breezes through all his school work. He does not appear to have any trouble with boredom, and is not a behavoir problem. He does enriched activities, like chess club in kindergarten, and reading news papers and chapter books, which he clearly understands conceptually, but his parents want him to have an age based peer group. His social skills are dead on for his age, and this is one happy little boy. I think that this family did things right. Your situation should be judged by you, and her temperament should play a big role in your decision, because I am sure that every child will not be as happy as my neighbor's son in a class where they are clearly way ahead, but it is worth wondering if his happiness is due to his ablity to socialize so well, in additon to having his academic needs met through enrichment, instead of promotion.
Sorry to be so windy, but I hope this is helpful.