Been a host and a guest many times. Been great every time.
Regarding advance notice: every single no-show (6) has been someone that has contacted me a couple of days to several weeks in advance. Invariably, their plans have changed by the time they get to me. I put on my profile that 24 hour notice is nice, but last minute may be accepted. If I can't host, I say I can't host.
Regarding background checks: the beauty of war showers hosting is you are never required to accept anyone. If it will be an inconvenience or something smells fishy, you say "Sorry, not available that night". Every time I get a request, I look that person up on warm showers and read ratings. (WS allows you to rate guests and hosts. Please use it for the benefit of others.) if I ever saw any questionable rating, I'd say no. I also usually google the person's name. If some Fraternity party scandal or other shady things came up, I'd say no. Most of the time, the person' strip blog comes up, which helps as well. (I don't see why it would take 48 hours to do those 2 simple background checks)
I find what others have written about guests and hosts to be the most beneficial info regarding who you end up hosting or guesting.