My criteria for reporting necrobumps is pretty much the same as that for reporting general spam posts. Is it worthwhile? Does it add new and relevant information to the discussion?

Often necrobumps in boards like Technical Discussion and Technical Support are fine. Since topics in these boards are usually about a discrete issue or question, then adding and building on what has come before is often better than opening a new topic on the same issue and having to repeat many of the same points and discussion. Necrobumps in boards like Bitcoin Discussion are usually the exact opposite. The vast majority of the time necrobumps in boards like this are just a couple of lines of spam, often repeating almost exactly what has already been said before, and very rarely adding any new information. If I'm reporting necrobumps on spam megathreads in Bitcoin Discussion for example, I'll also go back and report the OP and ask to have the thread locked.

Do you mean users reporting their bounty activities? The posts which are lists of Facebook and Twitter links? I see absolutely zero point in reporting those. There are thousands of posts like that every day; it would be a massive waste of yours and moderators' time. As long as they post within the designated bounty section and aren't spilling out to the rest of the forum, then just ignore them.

If you are talking about general spammy posts from users wearing signatures, then yes, you should report them as you would any spam post.