Correct. Although a doubled die coin can also display MD, when looking at the mint mark of Lincoln cents (before 1990) if you notice the same effect as you do elsewhere on the coin, it's generally MD. If doubling seems to be on all the devices except the mm, then look at it carefully.
At this time MM were hand punched into each working die. A doubled die will not show any doubling on the MM, unless it were an RPM.