I agree and do that often when I can't assign an aisle or window. The one catch with that strategy is in an oversold situation. If they have to IDB the person without a seat assignment may be SOL. As a Medallion, I don't worry about that too much but if I didn't have status it would be something to think about.
Don't they IDB the last person who checked in (whether or not they have a seat assignment) vs. the person without a seat assignment? That was my impression, but it might be wrong.