It depends on the track. If the source was stereo (drums for instance) you'll want the stereo file.
If it was mono (most bass tracks for instance) then mono will be fine.
I try to keep them separate.

You don't want to make a stereo source into a single mono track (reduces quality), but making a mono track stereo should be less harmful, so the safe way is using stereo tracks.

I may be different than many, but sometimes I intentionally make a stereo track into two separate mono tracks when mixing. Yet another option once inside the DAW.

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