For professional (or even amateur) singers, asthma (whether due to allergies or not) is especially tricky to handle because it affects the exact body organs they need to sing and they have to make sure that any medication or therapy they undertake won't leave any lasting damage or side effects to their singing capabilities. Avoiding the irritating agents that trigger allergic asthma symptoms are of prime importance. In instances where it cannot be 100% avoided, here's what Dr. Anthony Jahn advises (I've highlighted the important points):

For singers, I generally prefer to avoid inhaled steroids, if possible. Inhaling cortisone can have negative effects on the vocal folds. These include fungal infections, as well as possible weakness and atrophy of the vocal folds. While the latter two are rare, if the condition can be controlled by pills vs. sprays, there is less potential harm to the larynx.

Advair, a newer combination (steroid-bronchodilator) inhaler, is effective and convenient, but particularly bad for the voice. Even non-singers may develop significant hoarseness from this medication. Fortunately, the hoarseness usually resolves when the medication is stopped.

In mild cases, I have found Singulair to be useful. Its great advantage is that it can also help with upper airway allergies, but has none of the drying side effects of antihistamines. Of course, in some cases it doesn't work, but when it does it is easy (one pill a day) and helpful.

For more on how singers cope with allergic asthma, read the article Dr. Jahn wrote for the Classical Singer. If you're a singer and suffer from allergic asthma, consult with your allergist to find a suitable medication for you.