Read up on any of the etiquette books and they will say to tip housekeeping every night of your stay. I will admit that for years I never did and can't really remember when I started. Now I do. I travel a lot for business and about the only time I won't tip is when I am staying in a room for just a night when I have been stranded at some far away airport.
Mousekeepers are allowed to accept tips. I stayed at CSR last year for a convention and left a couple of dollars as a tip (I don't remember if I left it on the table next to my laptop or on the bed). I was leaving as the mousekeeper entered. She stepped out in the hall and asked if it was for her and I said yes (geez - I wonder how many times they mistakenly took money not intended for tips - oops. I have actually left $$ for a tip at some brand hotels and they maid did not take it). So if they are not allowed, she would not have asked.
You tip for service. The person who cuts your hair does not work for tips, and yet I bet most of you tip on the way out with your new 'do. I acknowledge that in this day and age, you can't take two steps before having to tip someone. That's why I always try to handle my own bags when I travel (although I never understood why you had to tip the skycap for not carrying your bags inside. If they grabbed them from the trunk, yes. But when you place them on the scale??

).
I used to deliver papers early morning. I got paid for the number of papers delivered and made about $200/wk. It was not a tipped position, but I always appreciated the tips I did receive (about 30-40% of my customers tipped). It wasn't much, but it was a nice supplement.
Mousekeeping is probably not a tipped position, like waitressing. Servers live off tips (and you'd think some of them would realize that!). But I also doubt they are pulling down the big bucks either. They work hard making your vacation more comfortable. You may or may not get extra service, but your acknowledgement of their work is appreciated.