My rule of thumb on mixing is as follows for stereo and mono tracks.

Stereo tracks: these are generally left as "centered" since for the most part they are already set up as stereo with slightly different audio information in the right vs the left side of the track. My main concern with a stereo track is the level of the track. A great example of this for me is the RB B3 tracks. Just use the track centered and vary the volume with automation as needed. That said.... Yes, you can pan the stereo tracks off center but you then start losing the audio data from the slighted side. If you really need to pan a stereo track, first, convert it to mono so you have 100% of the sound data, then, pan it where you want.

Mono tracks: These can be panned as needed or centered depending on the track.

regarding how far to pan the track. centered to 100% fully left or right..... nothing is off limits. Looking at the tracks individually probably isn't the best idea.... so I always look at them in the context of the mix. And it's kind of like an algebraic expression..... what you do on one side you have to do on the other. So, if for example, I have an acoustic guitar and I want to pan it 50% right, there should be something, perhaps a different guitar or a piano panned 50% left to balance the mix. I often use a pair acoustic guitar tracks, both playing a very similar, or the same part, recorded independently, and panned 100% left and right to give width to the mix.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.