Mono mixing - Virus TI Snow question

Most synth and sampler patches are stereo, but it's not always necessary to record them that way. It depends on the specific patch whether it sounds best just taking the left channel, folding the stereo output to mono, or keeping it in stereo. 

My primary hardware synth is a Yamaha MO8, which has stereo outputs labeled L/Mono and R. When I record it, I often just plug into the L/Mono jack and disconnect the R altogether. For things like acoustic and electric pianos, it actually sounds better that way, as well as sitting better in the mix. 
 
Bear in mind that many synth patches aren't really stereo, they're just chorused to give them an illusion of width. Recording just one side may not diminish the sound at all, and it will always be easier to mix.

A lot of thick stereo synth patches will muddy up a mix. In those situations where you feel you must record a synth in stereo, use the Channel Tools plugin to shift it away from the center. 

And yes, definitely set the track interleave to mono when recording mono. Also make sure the plugins you choose work correctly on a mono track. Not all do. Often, vendors provide separate mono and stereo versions of the same plugin for that reason, which is, IMO, the proper way to do it.

Channel Tools does everything MSED does and much more. I always use Channel Tools on every stereo track.

Checking your mix in mono is as simple as clicking the Stereo/Mono interleave button on the main bus.

Mono compatibility isn't hard to achieve. Just avoid stereo tricks like phase inversion and micro L/R delays and especially "stereo enhancer" plugins. 

I also like to listen to the stereo mix from a distance of about 20 feet away (as far as my studio dimensions will allow). In many ways that's a better mono-compatibility test because it simulates how people usually hear music from speakers, e.g. far from the sweet spot for proper stereo perception.