We dont normally leave a tip everyday, as in some cases we feel like they do "EXPECT" it and therefore dont EARN it.
The case of no towel animals on the one day that no tip was left is a prime example - had this happened to me they wouldnt have got anymore money out of me!
Mine and DH's feelings are that people get paid to do a job, unlike with waitressing where a service is being provided we do not tip. IF someone goes above and beyond, as our mouskeeping did and to be fair usual does in Disney we tip.
She left us varios towel animals, gave us extra detergent etc so we felt like she was doing something special, we tipped accordingly. Normally $5 every couple of days. And if we see the "usual" mousekeeper on the final day, we would give $10 plus any change we had left over - We also leave all food/drinks which are unopened and this year left a tinkerbell stroller which was only 2 weeks old and our daughters cinderella balloon from MK, which we couldnt fly with. She had told us she had a 6 month old baby girl so we made sure she knew it was for her, so no one else took it. (we got a double puncture on our Phil and teds and had to buy 2 strollers until we could get new tyres shipped to us) as we only had 2 children we could only take 2 of them home!