I decided to use back heat for my first rack but I will use belly heat the next time I build one.
My room stays about 75F in the summer but drops to just above 60F in the winter. Right now at 75F temps, the heat tape (at max recommended temp of 95F) is keeping the babies at 76-80F, but in the winter those babies are going to be mighty cold. I'll probably have to use a space heater in the winter.
Oh, and I'm using 4" heat tape, just so you know. If I had done it as belly heat, I would have used 3" heat tape.
I have heard of applying foil tape to the heat tape and the shelves to conduct some of the heat to the shelves, if the back heat isn't enough. I have yet to try that but it is something I will try before I settle for getting a space heater in the winter.