"It's considered 'bad form' if you keep bumping a thread just to keep it on top. It's like someone interrupting your conversation, even though they don't have anything to say after they get your attention ."
However, quite often, a question gets asked for which a clear answer has been provided in the past. Under those circumstances, I usually quote the nub of the original response, and give a link to the original thread. This sometimes rejuvenates the original thread by - as HotFlow pointed out - introducing it to a new audience.
Recently i have seen some people going back to the oldest forum topics on the site and making posts just to bring them back up to page 1. This is called "bumping." It is not a rule, that you can't do this, on this site yet, but what do people think about this?