Phase is hard to describe, but once you get it, you know it when you hear it.
A given track just doesn't sound right or act right especially when effects are added that should make it 'better' in a mix but don't. or sounds start cancelling out other sounds (I doubled the vocals but now some words seem muffled or have a weird chorus sound)
First test is, if you doubled it to another track and moved it, simply mute the doubled track .. does the problem go away? If so, likely a phase issue.

For panning, I try to visualize the stage.
I almost always use a stereo mix of the drums, whether they are generated or recorded live with multiple mics, just seems to work better for me.
On piano it depends; is the piano a central piece (needing more room/panning) or is it just a piano comping chords in the background?
If it is meant as a background comping I will likely split it to two mono tracks and position them both to have a narrower field, ie put the piano a little left on stage but still give it 'some' spread. Like 60% left for right hand and 40% left for left hand. Then it's still not 'MONO' but it is spread into a much narrower range of the stage.

Like J&B mentioned, this will vary from project to project and even song to song when you are mixing.

Hope that helps; there is a difference between having total control of two MONO tracks and simply panning a stereo track. So for any given solution it will depend on the application. No absolute truths.

By using two MONO tracks in the end, you can have more control over 'where' the instrument/performer is in the sound stage (and still control the L/R mix). But recording it in Stereo (or multiple MONO) tracks originally is what opens this option.


Make your sound your own!
.. I do not work here, but the benefits are still awesome