I know when exporting stereo tracks my track panning is reflected in the exported track. But when exporting mono, can I just leave what's panned, panned, or do I need to center all the mono tracks first?

I'd say it depends.
 
Depends on why you're exporting in the first place.
 
If you plan on bringing it into another project for further arrangement/mixing then it makes sense to leave them all centred.
 

Just as a follow up - when a mono track is panned say hard left - does this panning get saved in an exported WAV or MP3 file? I must admit sometimes I find the exported MP3 file to sound not quite right after the export and I always put it down to the MP3 compression algorithms.

Joden, that is definitely the question. And Bristol_Jonesey, are you saying that it depends because, each individual mono track exported does indeed bring with it its panning information?

Yes.
 
If not, there would be no point in panning at all if your exports ignored the pan settings.
 
Joden, I and many others do not export directly to Mp3 directly from Sonar.
I always export to wav first, verify that the export is as it should be and use that for Mp3 conversion

Thank you!

Question: (ok - 2) Does a panned mono track lose volume after export? Does the panning control change to match the original?

There should be no loss of volume. You would only hear a change in volume during the actual movement of the pan control, and even then it's dependant on which panning law you've got operating

If you export a mono track as a mono track, rather than exporting the same audio from e.g. master or an aux bus, surely it stays as mono? And being mono has no panning built into the exported wave because there's no "left" or "right" to pan it to if you see what I mean.

Panning a mono track is essentially telling the stereo bus it is outputing to where to place it in the stereo field, it has no effect on the audio in the track itself at all. Or does it in Sonar?

Yes it does and you can easily prove it.
 
Import any short wav into an empty project
Select the track and bounce it to mono
Pan it hard left or right
Export just this track selecting Tracks as the source
Re-import it and look at the wav