Always take the day off. There is no reason not to. You don't want to get sicker, get your training buddies sick, get the gym sick. Be considerate. My text here is meant in a gentle way, I'm not trying to sound dickish.
All I'm saying is we train what? 3 days, 4 days 5 days a week? No reason you can't take a day off every once in a while. Shit, even if I'm not sick and just feel too tired or feel like I need a day, I take it. Like BS said, "Listen to your body." Plus, rest is a very important component in this sport. But then again, so is consistancy. So you don't want to take a day off too often.
A little O/T but make sure you keep your diet when you're ill too (that is, the best you can).