The basic "rule", which of course is there to be ignored in your quest for musical glory, falls down to time-based effects and level-based effects.
Time-based effects, delays, reverbs etc., are normally used on busses.
Level-based effects, compression, EQ etc., are normally used at a track level.
Your question though, demonstrates the "ignorability" of this rule. Specifically, if you are routing something like all your background vocal tracks to a specific bus, there is nothing wrong with applying something like compression or EQ at a bus-level.
I guess, you could say that using level-based effects on a bus is ok providing that the input to that bus are of a like kind.
However, often times I will use Track EQ or multiband compression to fix individual track issues and then also use EQ on the group's bus specifically to fit the entire part into the mix.
Of course, all this is bullshit to be ignored in the pursuit of your sound. On of the defining sounds of the late 60's-early 70's is the so-called "New York Compression Trick" which involves routing all of your rhythm tracks, kick, snare, bass, rhy. gtr. etc., to a seperate bus, compressing the hell out of it (to make it big and fat and pumping), and then run it at a low, low level to really fill out the mix.
Confused? Just play with it and see what works.
Q.
