Would the mods prefer us to report the first off-topic post, or the latest one?

probably all of them

If you want to be precise, do the first and last posts, inclusive. Otherwise, either or, as long as you indicate.

Like Duh said, I think it'd be fine to report the beginning or ending posts in a so-called chain, so long as you mention that there are other related off-topic posts in that thread.

But don't feel obligated to report every post- we're neither blind nor stupid. Just point us in the right direction and we'll fix what needs fixing

Mentioning any other posts that need attention in the report is also good.