I'm about to do that today. Actually, I was sneezing and sniffling when I rode the day before yesterday; but I still got in 31 miles which is a long ride for me. I felt really worn out yesterday, and my head cold seemed a little worse. I don't get many colds so I can't tell if the progression was just how colds go or if I was tired from the previous ride. Today I still have a cold, but I'm going to take an easy ride along the Iron Horse Trail....no hills there! I figure I can always turn back, and as long as the cold crud hasn't gone into my chest there's no real harm. Besides, cabin fever's bothering me more than the head cold!
I would emphasize that this is a cold, not the flu, and temperatures here are 50 to 60 degrees. I think I would still (with a mild cold) try layering and not overdoing.