I warmed up my son's bottles only a handful of times during his bottle-feeding months -- breastmilk from fridge or formula. He didn't care if it was warm or not. I think it depends on if the child prefers it warmer or not -- some simply don't take it cold and refuse to feed.
My son didn't care so it saved me a lot of time.
Here's something from a website I googled about the subject ... storknet.com
Why do I need to warm my baby's bottle, and what is the best way to do it?
Many new parents believe the common misconception that a baby's bottle should always be heated. In actuality, there is no medical reason to heat bottles before serving them. Some infants may prefer warm bottles, but most will happily accept a lukewarm or cold bottle. Try starting your baby off with a cold bottle, straight from the refrigerator; if he takes it, you have saved yourself a lot of time! You will appreciate this for those late night feedings!
If you do choose to warm your bottles, always remember to shake the bottle thoroughly after it has been warmed and test a few drops of the formula/breast milk on your wrist before giving it to baby. This will eliminate hot spots and ensure the temperature is just right.