I also think that if you are serving something that looks like a commonly served item, such as pizza, but contains very unexpected ingredients, it should be noted. The flax-crust pizza is an excellent example. When most people see a pizza, they assume it is a wheat-based crust with regular cheese on it and they might not think to ask if it contains flax or almonds, even though they may be allergic.
Agreed. It would simply not occur to me that the dish in front of me that looks like cheese pizza is something very different.
Exactly. As the sucker who can't eat flax I generally try to be careful when I have a heads up.
For example, did you know that some people make pie/pastry crust with flax in the flour mix? It's less rare then you think, BUT I can usually figure out if I need to worry based on the situation.
If I can't see an ingredient list I never eat whole grain bread outside of my house. Some whole grain breads have flax seed. (Ditto pasta)
If I'm at a greasy spoon/diner type eating establishment I wouldn't think twice about asking if there's flax seed in the pie crust - there isn't.
If I'm at a local vegan restaurant (or vegan friendly, or generally ...'crunchy' type place) I will either skip dessert, or ask if they use it in their pastries.
I wouldn't expect it at a wedding because it's kind of an odd ingredient, BUT if I knew the wedding was catered vegan I'd be 100% more mindful of what I was putting in my mouth. Why? Because flax is a grain with a lot of protein, which is why it's used in vegan food.
You just need to let people know what they're dealing with.