Introducing foods one at a time - how far do you take this principle?

[Sanguine]

OK, so I've been reading. I understand the concept of introducing foods one at a time so that you can check if there is a reaction. But you surely can't do this for every food your child is ever likely to eat, can you? So how cautious are you supposed to be? For example, if you try broccoli and it's OK, do you assume that the child is likely to be OK on other brassicas? Or do you try out cabbage, cauliflower, etc. separately as well?

I did this with DD, as the books said - one food at a time, literally. With DS I decided that was pretty much nonsense. It's not like they're going to eat anything with a huge list of ingredients so you won't have any idea what causes a reaction. If I give him cauliflower cheese and he reacts then I know it's either cauliflower, cheese, milk, flour or butter! That would then be easy to narrow down later. Reactions to food are actually pretty rare, so I really think introducing two or three foods at a time won't be a problem.