I'll usually sweep around the lows-lowmids for some frequency buildup. Although it should be fixed in the mix, often it's a combination of several instruments builing up to peaks in the master, and it's quite hard to tell where to start, so then it's just easier to notch it down on the master. Actually I do this all over the spectrum, to see if there's any peaks at all, and then put those suckers down. I'm VERY careful not to overdo this, though, since I'm essentially lowering that particular frequency on all instruments simultaneously. I have had mixes turn from decent to magic by doing this, and other mixes don't need this at all, but it's always something I check at least.
Also, what you guys said, I kind of always put a highshelf in there, only boosting about 1-2 dB, highpass at 25-35 Hz, sometimes a minus 1-2 dB lowshelf too, since I tend to always mix so bass-heavy, haha.. >: