Mixing is a huge topic. It's also genre dependent.

BUT....

If a track is stereo, or mono, should depend on how it was recorded and predominately if it is an instrument capable of being a stereo source or a mono source. Drums, and keys, as well as orchestras are typical stereo sources. Bass, horns, fiddles, guitars, are often mono sources.

What I do. If a track is a stereo track, I will rarely pan it. It already has 2 sides for a reason. I will simply leave it centered and adjust the level so it fits the mix. Panning a stereo mix often simply looses one channel of the stereo sound. To alleviate this, you should convert the stereo track to mono and THEN pan it.

Mono tracks are easier to pan and place in the mix. When I pan something, to one side, I like to pan something to the opposite side at the same level to balance the sound.

Like I said.... mixing is a huge topic and more art than science but certainly a mixture of both exist in this thing we do. You get better at mixing by reading about it, learning the tools, listening and discussing mixes you like with others, and then of course, actually mixing. And don;t just mix your stuff..... mix tracks that others have done.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
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The magic you are looking for is in the work you are avoiding.