First sign of a cold I jam down the V-C, Echinechea and Zinc and head out for a ride. I find it helps ward it off. OTOH, if you have been training really hard you have put your body into a state of weakness and your immune system is a little lower than usual so you are susceptible to catching it. So I guess if you haven't been riding get out there, if you have been hammering hard rest a little. I think there are some allergens in the air right now since I have been a little scratchy but don't think it is a cold.
On the back end of a cold, i think riding is great when you are getting over it since all that hard breathing gives you the opportunity to hack out the bad stuff, and where else can you just spit without care!
(fake) Dr. Aussie
Disclaimer:
all medical recommendations are made from a position of absolutely no professional knowledge, mostly just hearsay, wives tales, myths and legends. No guarantee or responsibility taken if you damage your bike while riding with a head cold. If you hurt yourself or get worse, that's your own darn fault.