Just because you've worked as a production designer on big budget union films doesn't mean you can easily transition into being a post production supervisor. Different classifications have different skills that you learn as you move up the ranks. You don't always have to move up the ranks, plenty of film students come right out of school as directors of photography without starting as a loader.
It's up to the "industry standards" and it's also up to you. Do you feel you'd be a better director if you understood what all the other departments heads that you hire (DP, editor, prod designer, script supervisor, etc) do? Do you think you'd be a better department head if you'd already done the jobs of the people you're managing?
There are no right and wrongs, just choices for you to make.
For more tips and articles by top entertainment industry coach, The Greenlight Coach, visit www.thegreenlightcoachblog.com