Err, they do. Or at least, they vote on the rules at the beginning of every session, as you say.
I suspect the GOP will back down on the current stint of blanket obstructionism. Its pretty easy to see it backfiring on them (or rather, its hard to see it not backfiring). “We’re gonna stand up to the evil Health Care bill that has already passed by refusing to work past 2pm” isn’t exactly the most heroic rallying cry.