I recently went thru the same situation (b4 or after mix comp.) I usually apply my 2bus chain (1968 --> avalon 747) after the mix is done, just for a little sweetening and glue. Especially true for smaller ITB projects. a couple a days ago i decided to mix a song with the 1968 from the start. the mix was spread out on my board (dmx100) and i found that it was more coherent and tight than usual. Also, i felt that i was getting more out of the compressor while i was working. Of course i wasn't pushing it too hard, 3db reduction max, but i could feel what it was doing and work with it more than when i just patch a finished mix through the chain, otherwise when you do that, you have to be more careful and subtle. Also, as mentioned earlier, patching a finished mix thru a comp. will reveal all kinds of things that you didn't account for, and make you second guess your mix.
One thing to note though is that i had the song in pretty good shape ( i produce the material, so i kinda mix as i go along), and as soon as i strapped in the comp, i went to the loudest parts to gauge how hard it was hitting the needles (imagine that, a song with varying dynamics!!!). Anyway, once i had the levels, mixing went pretty smoothly.
FWIW I'm going to mix more often through the comp now, because it worked for me. I intentionally didnt mix thru the 747 as well, because i use that for the "added sweetening" at the end, I know that if i start with that, especially with the eq engaged, all i'm going to end up with is an overley bright and fatiguing mix..
BTW, all the above suggestion are purely for mixes that WON'T be mastered professionally!! yeah, i get a lot of those too
if i am sending the mix off to be mastered, then its digital straight off the board, with NO 2buss at all..
Hope this helped. Best of luck
