Just to add my two cents in, I think partly it's not something you should be overly concerned about. I think I big thing with mono'ing is that there's enough midrange information. Your stereo info should dip in volume, that's the point, but having a rich midrange will make them still sound apparent in mono.
If it is a huge concern for some reason, you can always add a mono triple of your rhythms in the center.
Take any golden age sneap mix (like killswitch or testament) and run it through an MS eq or something. Listen to just the sides, it's basically guitars and cymbals and vocal fx and that's it. Listen to the center and it's basically kick/snare/bass/vocals and the tiniest amount of guitars. They collapse fairly well to mono, but his mixes always have a killer midrange.