When my family and I stayed in two separate rooms at the AS Sports, we left a $5 tip in the room and the other half of our party left nothing. When we got back to the room that day, we had a towel animal and the other party did not. The next day, we both left $5 tips in our room and each of us got towel animals. We were so excited!!
When we stayed at GF the first time, there was only three of us, and we tipped $5 each day we stayed there, no towel animals or anything special.
When we stayed at GF the second time, there was four of us, and we tipped $5 each day. On one of the days, we came back to a hand towl penguin that my mom still has...she loved it.
Bottom line, I think a tip is needed. I have worked in the hospitality industry and the housekeepers, in my opinion, are the hardest working staff in a hotel. I always leave a tip no matter where I stay. Of course, as you can see from the three stories above, sometimes you get something special with the tip. I also left a little note each day to say "Thank You" or "Have a Magical Day"