My disc rotors get covered in all sorts of whatever. When it's been raining the bike even gets a spray down with GT85 and I'm not too fussy where it goes. They always seem to work fine, I guess a few hard brakes burn most of the crap off. When I do clean them it's with IPA, same as with all the fiddly bits. The calipers and frame on the other hand get covered in brake dust. There's no point cleaning it off because it'll just reappear next time I ride.