For sure they are gratefull.
I leave my 20 baht a day , sometimes more if there was mess left in the room, tip for cleaning stuff on daily basis. Leaving it at end of trip is bad idea for two reasons and I got it from experienced hotel worker in family.
First, specially if you are leaving on weekend, tip may be collected not by somebody who did a job for you day after day but by somebody who has part time weekend job.
Second, often checking room on check out may be done by staff from reception or somebody else , not actuall cleaner and that person may collect undeserved bounty.