If your allergen is everywhere, like good old corn, then yes this can be a problem. I assume for people with contact allergies to wheat, soy, coconut, etc. the issue is the same.
There is usually a thread going on shampoo issues on the Avoiding Corn Forum and it typically comes down to two solutions. Wash your hair with the one *currently* corn free shampoo/conditioner (ShiKai Natural Everyday Shampoo and ShiKai Natural Everyday Conditioner), wash your hair with a safe body soap or baking soda and use an apple cider vinegar (ACV) rinse.
It does cut down on your time in the beauty aisle, not that there is much in that aisle without corn anyhow. Lots of people on the board swear their hair never looked better after going to the baking soda / AVC method and it is also a great cost savings. No more wasted bottles of expensive organic super gentle formula shampoo looking for one that is completely without a trace of your particular allergen(s).
I haven't tried it yet, but I am getting curious. Here is the link to the Herbwife's Kitchen Blog. http://crabappleherbs.com/blog/2008/02/01/simple-herbal-hair-care She gives all the details on how to make the switch without looking like a mess. Sounds like something to start transitioning to during a vacation when you have a little time to experiment. Or for a New Year's Resolution!
Let me know how it goes if you take up the challenge! It may be happening soon on my head. I'll keep you posted. It's so dry in Colorado that, like most people with long hair, I try to shampoo every other day at most, so it may not be so hard to make the transition.