A couple of cases that I use to discuss.
1. Some posts are meant to be references, like character guides. Random posts asking X, Y and Z when X and Z are already answered in a previous one, and knowing the answer to Y could be useful to anyone using that reference, then I would definitely add it there.
2. Replying to someone's post from a year or more ago makes assumptions they are still around and have the same opinion. Or worse, I've seen people necro a thread to correct someone, using errata, sage advice, or just new books that were not out at the time the thread was active.
3. If you are asking a question and didn't search, a link to the older thread that answers it in detail is appropriate. Don't take it personally.
4. If the most recent thread is actually recent, say last post within the last month or perhaps even active - then continue that instead of creating another.
5. Between these a fuzzier area. If your topic has a unique spin, create a new one. If the most recent is six month or older start a new one - if it's newer you'll probably just get the same people giving the same answers.