We are "tippers". For Mousekeeping, we usually leave $1 per person in our room, plus $1 or $2 extra, esp. if we have our nieces with us. Our room can be chaotic. We also put the tip in a nice Thank You card with Mousekeeping written on it, so they know it is theirs. One year, we stayed at Port Orleans Riverside, and since we are from Louisiana, we put the tip in Mardi Gras cups we caught at a parade, with beads & doubloons. That was the year we got the most "freebies" like towel animals.
A friend of mine NEVER tipped at her hotel, has been several times, and wondered why she never got a towel animal. I asked her if she tipped, and she said no. I said that could be why. If you make the effort, so do they. But that's just my rationalization on it.
I do believe in letting CMs know how great they are. It could be the difference between a good day or bad day. Since I deal with customers of all kinds where I work, I know how it feels when someone tells you they appreciate what you do.