i think of mixing in mono as being like mixing on NS-10s (from what i've read about their use). you don't exactly mix so it's incredible, because as you point out, much of the audience won't be listening in mono, or on NS-10s. however, you know when it sounds good, you can trust it.
if you're using stereo material or effects, there's no way it will sound as good in mono, so don't set your standards too high for the mono mix. rather, keep checking the mix in mono, or perhaps start your mixes in mono to get your frequencies in line before having to rely on panning and stereo effects for separation. keep in mind that some stereo techniques are more mono-compatible than others (spliting frequencies between channels vs simple delay between channels). so not all stereo effects will sum to mono the same way. also keep in mind that for the most part, people will probably not hear the mix in ideal situations, so even the best stereo mix will not necessarily sound exactly like you've mixed it to the listener.
so yeah, don't get bent out of shape over it, but if you're checking in mono and it sounds at least decent, then you're on the right track. then, when you forget that your mono button is pushed in, and you get back to stereo playback, you'll be blown away by how awesome things sound. that's always a trip.
-marcus