If it is your own request thread, then yes you may. If it is a thread in the public/community areas of the forum, then only chime in if you have something particular to contribute to the thread. Don't just add, "Oh totally agree!" or anything that doesn't actually contribute to the discussion. If it adds to the discussion or if it is a question about the topic, then you are usually safe. Most will usually open with, "Not to necro this, but... My take/my question is..." that way it's clear that you're trying to get it going again rather than just adding LOL to something long dead.