It's been a while since I've checked so I may be wrong on this, but deleting your reddit account will remove the username while preserving the content. Deleting your comment without deleting your account will preserve both.

Reddit internally saves post deletions but not post edits, so if you really want to delete something you should edit it to some nonsense before you delete it.

Of course, the best way to protect yourself is to never post anything compromising in the first place because there are already a lot of services out there that cache everything, but I think it's a good practice to follow if you're going to delete anyway.