I use back heat for all my females specifically. I do have a belly heat rack that I use to put any large BPs that show a preference for stretching out.

My rationalization is:

I put the males and females with each other interchangably, hence my racks all of them except for one..have back heat. The reason for this is, I would not want to miss a laying and have the eggs fried because it got too hot.

The animals do not show any problems digesting with only back heat provided (sheesh..I gotta go clean out some more tubs tonight!)