High!
At first I must say that I'm not a pro, but I've tried a bit with the multiband stuff... Perhaps you'll get help from someone who has some real mastering experience...
At first, it seems to depend on what you wanna do with the multiband thing. If you use it on a mix, you might separate bands containg mostly BD, BASS,VOX/GIT and HIGHS. These can be compressed in the mix...
On the other hand, I like these multband thangs alot on individual tracks (voice and snare, e.g. -- I use
the TC triple c or the VS880EX MTK). There might be some nice stuff in the manual of
the TC finalizer express. They have three bands but if I remember it right, they have settings for different styles as examples. I'D have a look for presets that came with your comp (if there are some) first... Often its nice to have them to understand (in a intuitive, not in the intellectual kind) the way the multiband compressors work. You have to experiment a lot, if you don't have presets.
Lower bands might need slower attack and release than higher ones...
You might check out what's in the bands with an EQ first, just to get a feeling...
Try not to split the characteristic freqs of one instrument. If another sound triggers a compression in that band, while the instrument is playing at low volume, you may have breathing of one instrument, it suddenly ducks away...
Hope that I didn't confuse you more than I helped
Ciao
Axel