This is such a huge pet peeve of mine that I can't believe I haven't already written about it. I just did a quick search of the blog to be sure - nope, not there. So I'll begin.
I typically travel to about 4 conferences a year, and I generally share my room with one to three roommates in order to keep my costs down. I have no institutional funding for conference travel, which means that the money for this comes directly out of my pocket.
When staying at a hotel for more than one night, I was taught (by my mom: if anyone has different information, please share it!), the rules are a little different. For one night, I assume that I will have a clean room and that my bed will be made. I also know that the room will have to be made up again for the next guest, and so I do not tip housekeeping for this as there is no extra service being provided. (I am beginning to rethink this as more and more service jobs seem to be dependent on tips for keeping wages out of the toilet. Also, housekeeping is hard physical labor and certainly deserving of extra consideration.)
When I stay more than one night, I tip $5 per night to housekeeping, which generally adds up to about $20 over the course of the conference. At Jack of All Trades, Matthew Ryan points out that one could have a different housekeeper every day, so it is important to tip daily. (He also notes that, depending on the work the housekeepers do, he may tip up to $10 per day. To this I would add that if you are a slovenly and/or disgusting hotel guest - and you know who you are, so I don't need to go into graphic detail - you should tip at least twice that.)
In the past, however, I've had the privilege of staying with roommates who have not only never before heard of tipping housekeeping, but who, when faced with this new idea, are entirely resistant to it. And so, while this has only happened once or twice and not, thankfully, in recent years, there are times when I've been left to cough up everything save $2. Usually, though, there is no protest about the tipping, though these experiences have made me awkward and uncomfortable and anxious to ensure that a decent tip is left and that I am not screwed in the process.
But this is not the only source of roommate anxiety for me at conferences. The other is that my roommates are generally slobs. By "slobs," I mean that they leave their towels, wet or dry, on the floor, on the bed, or - and this is my personal favorite - wadded up and thrust far, far under the sink counter, so that the housekeeping staff will have to bend down and reach all the way down to pick it up. I suspect that this last is in an effort to keep the dirty towels in an out-of-the-way place. However, if you were cleaning 120 rooms in a day, how many times would you want to crawl underneath the counter to pick up the towels off the floor? And after how many times would your back begin to ache? Yeah, that's what I thought.
So, after my roommates leave for breakfast or a session or whatever, I run around the room, cursing them under my breath and picking up their sodden towels from wherever they have been thoughtlessly strewn. I usually leave them on the edge of the tub, where no one will have to stoop too far to reach them. And then I pick up all my crap off the bed - books, clothes, etc. - so that when housekeeping comes to make my bed, they won't have to deal with it.
(It is of considerable interest to me that these things happen so routinely, even though my roommates are different from conference to conference, in feminist academic circles in which we talk all the time about class issues, entitlement, and elitism.)