The clutter in our mind, heart, and body is the most important thing to cleanse prior to attending your first, or one hundredth, Tai Chi/QG class (Try below Sitting Qigong video to clear your head.)
In the long run T'ai Chi will help relieve allergy problems, but if you have heavy allergies and are heavily medicated, it may be helpful to lighten up on the medications prior to Tai Chi. That is, if your medications make your balance more difficult or make it harder to focus. However, never adjust prescription medication without your doctor's approval.
If you haven't tried acupuncture for your allergies, it can be a terrific nonpharmaceutical way to alleviate allergy symptoms with great results. Acupuncture treatments cannot harm, but can enhance your clarity or balance.