I don't use one, panning and EQ solve that for me
Like if you're mixing electric guitars (same amp, mic position or same amp sim), one on each side (LCR), EQ them slightly different so it sounds wider

 

I would *never* put any stereo or "3D" enhancement on the master bus. Instead of literally ruining the whole mix through one-effect-for-all, I would opt for processing selected tracks only, and pick the ones that benefit the most from stereo enhancement, panning, delay or reverb or whatever comes into your creative mind. Less mud, more focus. Good tracks I remember live from the fact that they don't use highly impressive stereo fx all the time, but just where they are most effective and appropriate for the listener.

Update: James, you might want to check out the January edition of Computer Music 2016, starting at page 48. Certainly a good read for you

 

Last edited: Dec 2, 2015