I highly recommend using a paddle attachment for making frosting, especially cream cheese or american buttercream. It makes for a much smoother frosting that pipes so beautifully. I find that whisk attachments often add too many large air pockets, creating a frosting with too many bubbles. If you have too many bubbles, just run the mixer on low with a paddle attachment for a few minutes or fold with a spoon/spatula at the end. This won't deflate the frosting - it'll just make it more smooth.