The housekeeping supervisor is responsible to ensure all rooms are cleaned to the resort standard, so tipping a housekeeper is not an oligation but a nice gesture. You do not need to tip or feel the need to tip to get a clean room. Especially, if housekeepers are paid above minium wage.
Typical guidelines are:
$3.00/night Upscale resort/hotel
$1.00/night at other hotels
If you stay for a week individuals might consider $5-7/night.
At your discretion, reward extra service, such as bringing more soap or towels, with an appropriate tip. Key here is extra and promptness.
Tipping overall is used to ensure promptness and if you are not satisfied with housekeepings work...don't tip or tip less, besides reporting the issue(s) to management.