I have always completely braided the night before and then put a slinky on over (I prefer the nonzippered ones that go over the shoulder). Usually have to redo one or two braids, but that's it. Have left them in up to 5 days during a circuit, but usually take them out and redo after 3 days. I have yet to have a horse do any damage to their mane by rubbing it.
You can bathe the horse as long as you don't have to scrub into the braids. If they are tight enough you can hose them down all you want, though, and soap around them, and not harm them (plus it cuts down on the itching factor if you get rid of the sweat).
If you can braid tight enough without using any gunk to sticky up the mane, it will also help prevent them from rubbing them out - that stuff is itchy to some of them. Good luck, and definately practice lots before hand; it's hard work