i do use a compressor on the guitarbus, to give the guitars more impact. it also helps to give leads there place in the mix as they push the rhythms down a bit when they come in. same goes for rhythms, when it's just 1 guitar playing (or 2 when quad tracked) the guitars pull up slightly, when the others kick in they duck a bit, especially with not too fast attack time this creates a feel of impact in my opinion.
i also use a C4 on the bus most of the time and not on individual tracks, keeps the overall guitartone more controlled in the low end range in my opinion.
my settings for the wide band compressor are usually something like this:
Rcomp
attack: 20 ms
ratio: 1:4
release: auto
make up to compensate the reduction, usually about 1 - 1,5 db with just rhythms, 2-3 db's when the leads kick in.
C4 settings are usually somewhat andy's style, tweaked to taste ofcourse