I never record the mix track. Like hi lo said, if you're making a mix of two sources that aren't in pretty much the exact same spot, its a good idea to align them in post to avoid echos or artificial reverb from the delay. Some people don't worry about this if the mic source is close to the PA, so the delay is minor, but personally even running on stage where the delay is pretty much nonexistent, I still correct for the very small discrepancy.
I'd tend to argue that at least some delay is a *good* thing, and can lend a sense of space. Also less odd phase issues that way. I would likely never "correct" a delay if the audience mics were on stage, and depending on the sound might actually add more.
That said, the mix track is nice for a quick reference (i.e., to throw on an MP3 player or to send somebody a clip), but I'd never use it for the final product. Obviously no chance for any sort of separate EQ/compression/etc with the real time mix.