Good grief, I can't believe the number of people who do not believe the maid deserves even a small gesture for cleaning up "their mess". The minimum would be $5 a day at a decent hotel.
We generally stay at Marriot Properties when we vacation and the daily rates are usually in the $300-400 p. day range. So I tend to think 5% isn't to much for the person who cleans up the mess I make. I also tip on a daily basis, which accomplishes two things. One, the maids do not clean the same room daily, and I would want a tip for good service going to someone who did not provide that service. Two, when tipped, a person generally will provide exceptional service for you in the future!