When bouncing a track I leave off all effects that aren't part of the original sound or aren't essential to the sound (usually reverb, delay, etc). If a sound that I'm using started off in stereo (didn't use a plugin to stereoize it) then I will leave it in stereo. The only time I personally sum something to mono before bouncing it is if I'm 100% certain that it will be mono in the mix (usually kicks, bass, snare, etc). I usually stick to doing it this way because it could be difficult to recreate the stereo image of a file that was rendered in mono.
You can pan a stereo track, but that might alter how its stereo image sounds.