Basically these guidelines can be considered as common sense. If you feel that you are not on same page with the main subject, then start a new thread. But if you'll discuss precisely same ideas, you can use the old topic.
Additionally, it depends on the thread; question, news or update. If the person wants to find an answer and the old thread doesn't provide answers that suits the level of understanding the user needs, bumping the thread won't help as many members won't contribute on the bumped thread. So, the user can create a fresh thread and as well refer to the old thread then tailor out what answer they're actually looking at. Hence if its a news or update it's not necessary because they must be changes in such threads. So, you made the right point by adding that it has to be a personal decision.