it just depends, i guess...i would venture to say that most people who mix with a lot of compression and limiting on the 2-buss are handling the mastering themselves...so they're really just getting that shit out of the way to start out
a lot of the "purists" out there will say to put nothing on the 2-buss if you're sending to an outside ME, while some of the biggest names in the industry(example: andy wallace) will shave off 8-10db with a buss compressor, then send off a mix that needs almost nothing done in mastering.
in the end, i would say to just do whatever works to your ears, and your workflow...personally, i usually have a compressor on the master doing some pretty gentle compression, and i'll occasionally turn it on and off just to hear what effect it's having on the mix