no typically the tubs do not damage the flexwatt. I have on old rack and just recently replaced the tape (no exact reason other than the wire was old and of poor quality) There was no abrasions scuff marks but that was all.

The back vs belly is a open topic of debate, to my mind there is no clear method that is superior. They are just different. Belly is direct so it takes less power to reach the desired temperature. Back is indirect heat, it takes more power but in the event of a T-stat failure terminal temperature will happen slower if at all.