good question. If the uth is controlled by a T-stat as it should really be there is no issue. If not controlled I'd not. personally I have used a sheet of glass 1/8 inch thick and attach the uth to this and set the tub on the glass. It is also easier to clean. the downside is the uth runs hotter than it would directly on the tub. In very cool rooms it may not be able to produce enough heat to get the hot spot correct.
I use UTHs on all my tubs with no issues. I would suggest putting foil on the sticky side of the UTH and then use foil tape to stick it on the tub. Makes the UTH reusable. =]
Ball Pythons are like Pokémon. Gotta Catch 'Em All!