I wash the protectors when they look like the need to be done and also when I have time - when full with 27 people, I couldn't possibly do them every week (or every other week) so on the weeks when I don't have all five to change over, I select the ones which look like they need it. The weather also makes a big difference ...
When I stay somewhere, I always strip the beds and leave it in a heap somewhere. I ask people to strip the beds here and sometimes the pillow and mattress protectors get put in too. Sometimes people fold them neatly, which looks really nice, but also takes more time unfolding them for the wash
It's nice when people make an effort, so I'm not complaining at all...