Tipping the housekeepers at any hotel is considered appropriate. The amount you tip is certainly up to you. Somewhere around 2 to 5 dollars a day is standard. (Based on the number of people staying in your room, how much you tidy up after yourself, etc.)
Of course, if you have a lot of special requests, or you feel they did an exceptional job, a higher tip is always appreciated.
If they didn't do a good job, then of course I wouldn't leave a tip. And I would contact the front desk and make sure the situation is corrected.
I usually tip daily, as in most hotels (especially large resorts like Disney) you'll probably end up having different housekeepers during the week. I always leave a small note with the money in a visible location (edge of the table, dresser, etc.) so that there is no confusion that the money is a tip for them.
Hope this helps, and enjoy your trip!