I agree with all of this. But you have to watch the caterer like a hawk if you have complex allergies. I still remember the time I said "no wheat" and they served me couscous. I tried to explain that couscous are a pasta and thus made out of wheat but they did not get it. Caterers may not be the ultimate experts in food allergies.
I'll second this cautionary note, especially if your allergies can be life threatening. My brother has very serious (and extensive) food allergies, and has been almost killed a couple of times by food service workers who were either careless or simply unaware.