We leave a small tip for the Trash and Towel, usually a few dollars, and a larger tip when we leave, last time I think it was $20. If you look at how much work goes into cleaning those units, I think it is quite amazing. Personally, I believe that, in general, the housekeepers do an excellent job and deserve a tip.
Also, the general rule when staying in a hotel seems to be $1-$2 per night. So, when I look at the amount of work to clean a 2 bedroom at OKW compared to cleaning a normal hotel room, I still think they deserve at least $2 per night.