Never had issues? I have my tubs directly on the UTHs and in a rack I have set up there will be no ventilation per-say for the heat meats and heat tape. I've always heard it was okay to place tubs directly on top of the UTH as long as it's regulated correctly. Especially since I like to thoroughly clean my tubs, I don't like having to rip off tape and then secure it again.
Sorry to steer away from the original posters.