Some people tip at the end. However, we belong to the camp that tips daily, since the same person may not clean your room each day. Also, we tip daily in hopes that it will encourage the housekeeper to do something cute like pose our animals or shape our towels into animals, etc. Sometimes they do, sometimes they don't - I'm sure it depends on how rushed they are and how much experience they have.
However, after our June trip I posted under Disney Tips a warning to directly hand your tip to your housekeeper if possible. Our tip was taken one day by someone other than our housekeeper. We just happened to return to our room and notice that the money was gone. You might go over and read that to see what everyone's responses were. There are many postings about tipping.