People often play louder at performance time than soundcheck. Actually, people always play louder during performance, and tend to back off their mics, as well.
A lot of times I find with small PA boxes they have a midrange bump to help the vocals. My board tapes then have guitars too loud, as I have to boost them a bit to get them to come through. I also cut a bit out of the mids in acoustics to make a little spot for the vox.
As far as the headphone thing, I find the same problem. I have to boost the guitars so I can hear them in the cans over the PA, and they end up too loud on tape. If you want to be sure, go somewhere quiet after soundcheck and listen to the recording. Eventually you will know your gear well enough to get it sounding good on the fly. At this point, I know if the vocal sounds X amount louder than the guitar in my phones, the tape will sound good.
It does take practice. These days I can make a decent board tape w/o sacrificing the room sound, unless it's a loud band in a small club, and there are only vocals in the PA anyway.