I cancel plans if I don't feel up to going out or if something came up, but I always try to give an appropriate amount of notice. If I don't feel up to it, I make up a reason that sounds better (ie, lie) because I don't want the person to know they're getting canceled so I can sit on my couch and talk to you nerds all night instead.

I don't mind if someone cancels plans on me, because I can usually find replacement filler for the evening.

Something I find even ruder than canceling, though, is not giving enough notice - whether it's canceling or rescheduling. If you can't make a date and you text me ten minutes before, that is unacceptable. If you try reschedule with me from 4pm to 3pm and it's 2:40pm, that is unacceptable.