How many inputs does your USB sound card have?
Generally, this all depends on who's mixing and where. I would recomend that you take the line off the board and put a couple of mics recording the room as well.
The sound man's job is to mix the band so that they sound good, this does not always mean that every instument is comming out of the board in the same way that they would in a studio setting. The amps and drums bleeding off of the stage themselves are also "in the mix" so some instruments may show up on a board recording quiter than you would normally want them to be. In some cases you may end up with really loud vocals and barely any guitars depending on the situation, but the mix in the room sounds just fine.
Depending on your situation, and how cooperative a mood the soundman is in, you can get him/her to send you a mix using an aux (if there is one available) and a set of headphones to monitor it.
I still recomend a couple of room mics if you have the tracks and inputs though.