I'm not one to usually go against a Pediatrician's recommendation, but I would personally recommend starting the EI process sooner, rather than later. Our Pediatrician recommended it at the 15 month check-up, as our son was not saying anything other than non-descript babble (no Mamma and no Dada). We have just gone through the EI process and I couldn't be happier. There is no cost and no risk to calling to have your son evaluated. In fact, even if she is deemed "within normal limits" for speech development now, you have rights to have her evaluated in the future.
But the most important question you asked is "when should you worry?," and I say try not to worry at all! All kids develop at their own rate, and it all evens-out when they are a few years older (it really doesn't matter when they first walked, talked, etc!) If you wait, she may start talking by her 18 month check-up. If she's not, you can call EI for help then. 18 months is still very young, so getting help will still be optimal!
Please feel free to let me know if you have any questions about our EI experience.