I worked at a Comfort Inn through college. I was grateful for any tips I got, but I didn't expect them. I will say that I was paid minimum wage, and the work can be back breaking. I know for the first few weeks, I was very sore, from making the beds, and making hospital corners on the sheets, cleaning the bathtub...etc...
I really resented people who seemed to enjoy making messes on purpose. Like food on the rug, or beer bottles and junk food all over the place. I did not mind a sick kid, or an accident, but some people were disgusting, and then expected that it would be spotless by a certain time.
It is not an easy job, and I have always tipped housekeeping since my experience with it.