If there is no cleaning during the week, we don't leave a tip. Even if not required to, I make sure all dishes are washed and put away, that the beds are stripped and sheets piled near the front door, and all towels are put with the sheets. We empty all trash and sort recycling (when they have it,) placing it in proper depositories . In other words, we do much of the work that they would have to do, so I consider our help sufficient. We also leave all unopened food with a note that they may take it, and try to leave at least two cold sodas in the fridge for them.
If there's a midweek cleaning, I leave $5. If the midweek cleaning was extraordinarily good, I add another $5 to the $20 we customarily leave at the end of the week.
On very few occasions, we have not tipped because the "housekeeping" service was so deplorable that they left dirt unattended and took towels that they failed to replace. Minimum wage is no excuse for sloppy work.
BTW, this has never happened to us in Mexico or in the Caribbean - the housekeepers are eager to please and meticulous. We have left as much as $5 per day for them. They leave extra towels and make animals out of them on the bed, and strew it with rose petals in a lovely design. What a nice touch! That took some extra time, and it's worth it to me to pay for. It's only in the States that we have found lazy housekeeping staff.
Cindy makes a valid point that the generosity we can muster for housekeepers makes for a much friendlier atmosphere. As she said, it's not much to us, but it's probably a lot to many of them, particularly in less affluent countries. Rarely is generosity wrong.