This question comes up around here from time to time. The fact is that hotel housekeepers (at Disney and elsewhere) generally earn a very low hourly wage. Part of the reason that hotels can get decent housekeepers despite paying such a low wage is that a good portion of guests tip the housekeepers.
Most tipping guides recommend a dollar or two per day per person, but the statistics I've seen indicate that only a minority (a significant percentage, but still a minority) of guests actually do tip housekeepers.
It is true that, when it comes to federal minimum wage laws, housekeepers are not on the list that can be paid a lower minimum wage because of expected tipping -- waitstaff, of course, can legally paid a lower minimum wage. I wouldn't use that as my guide, however, becuase I don't think it's the government's job to tell me who to tip.
The upshot is that tipping is a matter of preference. It is a fact that housekeepers expect tips from a substantial number of guests, but that if you don't tip, you won't be alone in not tipping.
In answer to the OPs question I usually leave a buck or two a day, when I remember. When I don't remember, I don't feel guilty about forgetting. I do try to leave it each day, rather than all at the end or all at the beginning.