It wont hurt the belt to just check it. just a little squirt will tell you real quick. Belt dressings leave a stick residue behind. And sling junk all over the engine bay. If the belt is cracked and checked it needs replaced any way. So no harm.
I build custom cars for a living and would not give out bad advice. I see enough of it on the net. Its a simple check a guy can do himself without much trouble.
Tim