I agree that working can lower your ability to devote resources to healing; however; call me compulsive, but unless I'm falling down, passing out, near dementia sick...I go.
I find that exercise can clear my sinuses when they're clogged, and a good steam aftewards will really help any cough you may have.
But I absolutely agree that if you're sick, you're best bet is to stay in and rest. If you're compulsive about it like I am, you could probably manage a light workout without doing too much harm.