This is one of the knottiest issues in engineering work. What constitutes a full revision? When do you issue a partial revision, if you do at all? How do you track the changes? How do you roll them back if necessary? Worst of all, different shops use different criteria, and sometimes the same shop uses different criteria for different projects.
It seems like common sense, since storage is so cheap these days, to save a new copy of the drawing for each revision, and append the revision number to the file name. That way the old revisions are still available if you need to refer to them or send them to someone. Even better, keep the revisions in a subfolder so they don't clutter up the main project folder. And this should go without saying--keep current backups!
The current working file should have the project name (or whatever) with no revision number. Again, though, some shops will use different standards.
For smaller projects, there's no need to keep old revisions, especially if you've sent them out all along. You can go back to those emails for attached copies of old drawings.
As always, simpler is better. Make a plan and stick to it. Otherwise you're asking for trouble.