I don't use it for formula. Room temp is more convenient, especially when you are out! I wouldn't use it unless your baby fusses. Don't add more work! I warmed up breastmilk to room temp when I was pumping.
I agree with pp - don't warm if you don't have to. Room temp is easiest. I have a Dr Brown warmer because I pre-make 24 hours worth of bottles for my twins and refrigerate them. I only use the warmer to get them back to room temp. Sometimes I just throw the bottles in a cup of warm water for 3 min or so to get them to room temp as well. When we are out, I bring refrigerated formula in a cooler bag and a cup to put warm water in. Or I just make a fresh bottle using bottled water and no warming is necessary.
I found if I didn't warm the breastmilk the fat wouldn't mix in & would stick to the sides of the bottle. So that's the only reason we warm. I don't have a bottle warmer just a cup of water heated in the mic & tgen we put the milk bottle in the cup.
My lo gets my pumped milk right from the fridge. No problems getting it to remix, just about 30 seconds of gentle swirling/shaking. She's never complained yet, and she still breast feeds fine from the breast and gets it 'warm' too.
Re: Warming milk
I wouldn't use it unless your baby fusses. Don't add more work! I warmed up breastmilk to room temp when I was pumping.