I'm fighting a bad cold right now also and I will not ride. I don't like all the runny nose I get when I ride without a cold, I don't even want to deal with that with a cold while riding. Plus, my energy is so low right now, I just want to sleep and the chest congestion is awful. If my body says sleep, I sleep (unless I'm at work). I think it's OK to exercise if you're feeling a little under the weather, but I also think it's really easy to over push yourself and delay getting better.

I'm basing this on only my personal experiences, there could be good studies that prove otherwise!