Don't delete threads unless there is a reason. Someone may have the sane query as you in the future so if is beneficial to keep as much information available as possible. That's the amazing thing about the Internet, every time you post its indexed by google so we are building a huge database of knowledge.
I tend to go back and mark old threads which have gone stale as resolved...folks can still read and post to them, but when a discussion has run its course it helps keep me organised.
MOD NOTE: FWIW we'd much rather mark a thread resolved (you can do this yourself) and close it to further discussion (we need to do this) than delete it in most cases.