Due to a medical issue wherein I had to be on some unresearched (for breastfeeding) drugs when I first gave birth, I was unable to breast feed. I pumped and dumped as much as I could, but ended up having to do only formula, so we too had some of the same questions when we were starting out.
I agree with the other two ladies here who posted. Specifically, we do the same as Charlotte did - we don't heat the bottles at all, we just make the bottle with room temp bottled water. Which is also a plus for when we're out and about, he's not used to heated food, so it's no big deal when he gets it at room temperature.
Yes, if you were to re-heat it after he's been drinking it, bacteria could grow. Just toss it after 24 hours if it's been in the fridge and if it's been out for more than hour, also toss it. If he's drank off of it at all, technically, you're supposed to toss it after 1 hour even if you refrigerate it, but you're right that some people don't. Again, the reason is bacteria.
As a side note, they also say not to heat formula then put it in a thermos if you're taking your baby out for the same reason - bacteria could grow inside the thermos. We usually put the scoops of formula in one bottle and the amount of water needed in another bottle, then when he gets hungry, we just quickly mix the water into the formula bottle and shake it up - good to go. You have to figure, your breast milk isn't heated, it's at about room temp, so they're used to it anyway.