With a multitrack format, panning is less important at the tracking stage. I tend to leave most instruments centered as it is distracting to the players to hear themselves off to one side or the other in their headphones. The exception would be instruments that are being mic'ed with a stereo image in mind - particularly drums. Make sure that you pan from the drummer's perspective (i.e. hh panned left, floor tom panned right, assuming a normal set-up) as the drummer doesn't want to hear the headphone image reversed from what he is hearing live. (Of course, he could just turn the headphones around!)

If you don't pan anything, you have, of course, a mono image. In your case, since you are mixing on your analog mixer on the way into a stereo soundcard, you are essentially mixing and tracking at the same time. Many very famous recordings have been made this way - it is usually called "live to 2-track" recording. But in this case if you don't pan anything while tracking, you will be forever stuck with a mono recording. (This all assumes you are recording everyone at once, and not one instrument at a time.)

Typical conventions for panning in pop music are kick, snare, bass, and lead vox down the middle. Everything else moved away from center at least a little. Panning is a way of creating space for the different elements. Classic example is putting two rhythm guitar tracks on opposite sides. Naturally, you can break whatever "rules" you choose. Everyone has their own variations: e.g. background vocals panned wide vs. tightly bunched. Another example is some people love the stereo image of drums or acoustic piano panned wide as possible, which often creates a virtual 30 foot wide kit or piano (depending on how they are mic'ed). Other people find that very annoying.

That's why you'll always see people here saying:
YMMV. (Your mileage will vary). Best way to learn is to try all the possibilities at least once!

You will use panning in the rough mix that you feed to players at the over dub phase. But I wouldn't worry about panning in the initial tracking stage, and in most smaller live venues like bars and clubs the shows are mono to deal with the fact that alot of the seating, dance floor etc are not in the "sweet spot".When you're performing in concert halls panning may become an issue. You could use panning to get a different headphone mix for different players while tracking, if you had to, but there are better ways to do it.
When mixing your tracks, panning is the tool to create some interest and spacing in your mix. Tkae care Logan

During tracking on some consoles, using the pan knob becomes necessary. For example: you are recording a drum kit to multiple tracks using quite a few mics. Say you have 3 rack toms & a floor tom with 1 mic on each. A common method is to group all 4 toms so that they are recorded onto a stereo pair of tracks. You would need to assign each input channel to the same 2-buses or subgroups & then pan the individual inputs across those buses or subgroups to create a stereo image of toms across the stereo pair of tracks. The pan knob would tell the signal which bus or subgroup to go to.

In the mix, panning not only gives separation amongst instruments by providing each instrument its own space in which to sit (which promotes clarity of the mix), but can also be used to create interesting effects such as movement of an instrument across the stereo field or alternating "appearances" of an instrument from one position to another.

I have even heard (and used from time to time) panning being implemented to create odd or spacey effects on instruments by panning the dry signal to one side & its effected signal on the opposite side.

Panning is a very useful tool in the entire recording process. Experiment with it & have fun! :w: