The rule of thumb to follow here is whether your adding to an old thread is appropriate. If you're dropping by to say 'cool' or 'probably' to a months dead thread, then you should definately not 'necro' bump the thread. To be clear, this is a form of bumping which is against forum rules, both here and almost universally across the web.
However, if you were about to start a new topic for a question, request, or substantial update and came across an old thread on the same subject, then it is perfectly permissable to necro.. In this instance you are doing the forum a service by keeping the information and discussion of a particular subject organized within the same thread. In short, it helps in keeping the forum organized.