What you should prioritize in any paper, mathematics or otherwise, is readability. Organize the paper so that your reader can comfortably follow your argument without a lot of jumping around.
In a short, relatively "flat" paper, almost any organization will probably be ok. Flat in the sense of later parts not depending fundamentally on earlier parts.
But in a longer paper it becomes an issue, for me at least. Things should be introduced with some context: Why is this being introduced here. If I see a definition, I expect to be able to easily grok why it is here from some prior context or by being used quite soon in the development. A definition or piece of notation introduced without context is just irritating.
As an extreme example, imagine a calculus or abstract algebra textbook in which all of the book's definitions are in the first chapter along with every notation that will be used.
I'll note that the very purpose of defining things in mathematics is to give us something to think (and write) about. Defining something with no context is just noise. We define rational number or Abelian group for example, because we want to say things about them. If you define them, but don't soon discuss them, the reader has no context.
So, first think about the flow of the paper from the standpoint of the reader. I think that in most work with any significance the "just in time" organization will probably work better. I'll admit there may be exceptions. But it is the readability that should be prioritized, not some abstract concept of an "ideal" organization.