Thanks everyone for your input. Apparently opinions on this are quite varied.
When staying at any Disney resort we've always tipped about $4 per nite, leaving the tip at the end of our stay.
We got back from our 1st DVC trip in September and I hadn't thought to ask about this before going. We stayed in a one bedroom and when we arrived room was in good shape, but had the bare minimum of towels (I'm sorry but 2 washcloths for 2 adults for 4 days is just ridiculous!), tissues (an almost empty box) and toilet tissue. My husband had to actually go to the housekeeping cart in the hall in the morning for toilet tissue.
Surprisingly, on our trash & towel day, we were left with an overabundance of towels. I think this was to ensure a good tip. The trash was all taken care of as well as the used towels. When we left that room the next night, we left $10, but I felt like it wasn't enough.
Our last night, we moved to a studio which had a dirty microwave. No tip for that night.