Do you change speed on the Dormeyer by shifting the brushes? I once had a Hamilton Beach Home Motor that could run in either direction by changing the brush position. I'm guessing shifting the brushes is speed control because there would be no value added in being able to run the mixer in both directions.
As long as the appliance is in good operating condition, I'd say it's better to run it occasionally to keep the moving parts free than to let it sit and have things get frozen up.