mono, stereo or two mono tracks?

[bobbo]

I've been wondering about this question, so here I go. I record guitar, bass and vocals in mono most of the time by selecting mono in the recording VU meter before recording. When I am sending these tracks out of my soundcard and into my mixer for some effects and EQing etc, should I be playing back these tracks that were recorded in mono, in mono or stereo? In the preference section you can choose between mono or stereo playback, I usually pick mono playback so I can get better performance.
Should it be mono or stereo playback when going back through the mixer and back to the soundcard for a mixed version of those tracks?
Also, has anyone ever tried using the mixdown function in ntracks, by mixing down two identical final mixes? I tried this once, and it really seemed to sound good at the time, louder, fuller. I just did it for fun at the time, and forgot about it and never tried it again. Just thought I would see if this seems like a bad thing to do.
Thanks
bobbo