Should I clean the toilet with my roommate's washcloth?
Probably not. Is there a reason why you want to make your roommate upset?
No, I don't even know why I want to.
It is pretty messed up to use someone else's washcloth for this purpose. Not only are you throwing away their trust, but you are also risking their health and sanity. This is the type of action that could thwart your roommate to want to find a new place to live.
I never want it to come to that.
But it will if you do that, people don't like it when you use their things without their permission. Also that could mess their things up and they could want it replaced.