I read one post that says the longer and faster you run your mixer the smoother the buttercream. I read another post that says running your mixer too long and at a high speed causes air bubbles in your buttercream. Which have you found to be true?
I think if the buttercream covers the paddle the more you mix the smoother it gets. On the other hand if it does not cover the paddle then the more you mix the more air that gets in.
It may depend on the recipe. I'm one that believes beating longer is better (totally pulverizes the fat into teeny tiny micro particles, which makes for a smoother icing) and my icing doesn't cover the paddle.
I would say try beating your recipe for a short time and see how that works. Then beat it a little longer....test it ... beat it longer ... test it.
See what works for you.