Hi Mike,

Generally, I delete them, unless I am time-tracking. I like the finality of deleting after completion, and it suits the GTD mindset. If you need to track tasks in some way, or just really like to see those little checkmarks that mean done, you could move them to a completed tasks section — if you want to, you could even separate them by project tags to make for easy sorting of multiple projects’ progress.

Hope that helps! :D