When I'm travelling it's usually just me and only for one or two nights so I typically don't tip at all. If something extreme happened (bloody nose that dripped on the floor/sheets/etc or a friend happened to get sick in the room the night before) then I generally tip a couple of dollars just because of the added hassle of having to clean it up. The way I personally see it, keeping the room clean is their job, and they get paid non-commission, hourly for it, so if I'm not requiring anything special, then they are already being paid and there's no need for me to "encourage" them to do their job.

On the few occasions that I travel with family and there are more needs from housekeeping (extra soaps, towels, etc) then I generally tip 3-5 bucks/day.