Bottom heat is just a drying if not more than overhead heat. Sure, the air can be humid, but an animal laying on belly heat does get dry. So you can't assume that overhead heat dries out a cage but belly heat doesn't. Both do.
In my own experiments with heating methods, I came to a conclusion that exotics like boas/pythons do very well with overhead heat if the room is generally cool. I swear that my ball pythons did better under ceramic bulbs than over just heat tape alone. This was at my house. Could be different for others.
If your room is already 80 deg, then you are good to go with a strip of Flexwatt set at 85-88 deg. I'm talking for boas/pythons. Colubrids are fine at 80 deg.
For a boa, it is not good to have a warm heat pad in a cold cage. But like I said, if your room is 80 deg, then you are in good shape. I don't know why I'm assuming a boa/python cage here. I guess because heating colubrids is less of an issue.
-----
Mark 