I always have rain gear handy in case the need is there... My thought has always been, if it's pouring ass rain when I pull in, I'll just get it parked and then see if it lets up.. If it ain't gonna let up, then pull on the gear or decide if I'm even staying there... :)
I've left early for weather before, but have been lucky on at least setting up.
Different parts of the county makes a huge difference too.. I'm in the NW, so you know basically what to expect for the most part.. If I went to the SE, I'd have no clue! ha, ha!
Good luck! Mitch