I agree that if you want to sound live, then stereo is the way to mix. When I mix my backing tracks, I make sure to pan when I have multiple instruments. The difference in the live feel is huge. Just listen to any commercial record through a set of headphones and you will hear how much they pan to preserve the sparation between instruments. In most cases a panning of 40-50% is enough, which means you still get some sound in the "unpanned" speaker. I'd rather have someone dance in front of me getting a live feel as they move from one speaker to the other, than to lose that in a mono set-up.