everything under 600hz need to be in mono, otherwise the tune will suffer when etched to vinyl or played on a mono system.
as mentioned above, its good idea to have hi hats and cymbals wide, since they mostly live in the higher frequencies. if you feel that a lower element needs to be wider, snare, toms etc, you can do that with their effects. For example, instead of panning the actual snare, keep it in mono, then send it to a stereo reverb and cut all the lows of it, so you get the powerful mono lows, with a nice wide top end.