I am an uber paranoid person. I follow the rules and read the instructions

A heating pad says "ONLY" for use on glass.. but i see folks using them on plastic tubs.. is there a certain type of heating pad for tubs?

I have realized my obsession with balls is growing to more than i have room for with tanks.. hehehe.. and i will need to start using tubs. The idea of "breaking the rules" of the heating pad is something that concerns me. o.O