Since my EP350 keeps going into stand-by at low volumes, I have a 'special' setup until I get my EP500. All my speakers are set to small with an 80Hz crossover, but send the bass to both the front and sub. This way I still have most of the bass when only the mains are playing, but get the extra grunt and extension from the sub when it does kick in. It works pretty good, but would be better if the sub worked all the time (hence why I'm upgading).
As for a general rule, there isn't one (that I know of). I believe THX standards are 80Hz crossover for all channels, so start there and play around with it until it sounds good to you.
