Anything sticky in general isn't good in an enclosure. I personally use adhesive velcro on the back of my thermometers to attach them to the back of my tank and have no problems with them. My BP even hangs from the thermostats sometimes and they don't budge or move at all. There's absolutely no way he can find a sticky part of the adhesive so I don't worry about it but I wouldn't advise other people to do it just because they might not have the same experience as I have.