Well, yes, even for dental work - any kind of invasive surgery, really - they're afraid the presence of bacteria or virus can complicate the procedure and cause or worsen any inflammation or infection. I think that if this phase of your cold has already passed, though, meaning you no longer have an inflammation/infection and the swollen glands and congestion are just stragglers from the cold you should be fine. But I'd just call and check. Hope all goes well, whenever it goes!