Your inbox should only contain 'tasks', that is actionable items that are on your list of things to do. Everything that is in your 'Inbox' (including sub folders) should only be to-do items. So do it now, delete all emails you have done, or move them out of the folder if you want to keep them for reference.
Figure: Good Example - Everything in your Inbox (and subfolders) is still "to-do"
In fact you could go so far as to say you should not do anything unless you have an email telling you to do it (which is why we send ourselves emails).