Well, it's technically "against the rules", but since we all know how precious breastmilk is and how expensive formula is, there was no way I was going to throw away 4 or 5 oz. of formula if baby decided that she didn't want any after drinking an ounce. Back in the fridge it goes -- no harm or foul. I know that there are bacteria and whatnot there, but I guess baby's body has figured out how to ward it off (I guess those first few days I was able to breastfeed and she got some colustrum in her were good!
). If there is only an ounce left, or if the bottle has been out for more than about 90 minutes, then I DO dump it.
Oh, and I never heat up her bottles... we've always given them to her room temp or cold straight from the refrigerator with no problems. My mom thought it was weird until I asked her how she drinks her milk... "Cold!" she said. "Exactly!" I replied.