If you're in a restaurant with anything you're allergic to on the menu, you should mention it to your server. For most of the population, cross-contamination isn't going to make a difference, but it can for those of us with food allergies. There's no reason why you should have to pay to be itchy.

I'm allergic to seafood and nuts (among a gazillion other things) and even though I always order conservatively, I do mention it and haven't yet had any grief (or reactions!) because of it.

One one particular occasion where I had to go to a restaurant that was basically my worst nightmare from an allergy perspective, but had to attend the dinner for business reasons, the chef even came out to talk to me about his proposed dish and said he, "welcomed the challenge."