A study cited by pediatric nurses found that babies tolerate room-temperature formula as well as slightly heated formula. It is recommended to use room temperature formula in a bottle because it removes all concern of burning from over-heated bottles.
Two recommendations if you choose to warm a bottle: First, bottles are best heated under warm water. Do not use a microwave to warm the milk. Microwave heat is uneven and increases the chance for burning. Second, be sure to gently shake the bottle and then test the temperature of a small amount of the formula on the inside of your wrist before allowing your baby to drink.