I have done this with my work beater with well over 200k on the clock but the Jeep gets dirty and then washed. I rode with a local jeep club Saturday and it took six hours cleaning to get my mall rating back.
I find this doesn't work too well on jeeps. For one, when it dries, it'll be a mess of water spots. For another, the water alone usually isn't enough to wash the dirt off. If you haven't applied some sorta surface protection (wax, sealant, etc), stuff will stick (especially tree sap), and that's even worse.
If you're about to get a huge downpour, at least get a bucket with some soap and water and apply that to the jeep, if it rains enough to wash it off, it'll be pretty clean I guess. Despite being rugged vehicles, I find the paint isn't that great. Ironically, the paint used on the body color fenders/top is particularly delicate. I don't think their paint method bonds to plastic that well.