When I started tuning my own skis a few seasons ago, I actually managed to reduce the the edge angle over the course of the season because I had failed to to plane the sidewall. When I took my skis in to be stone-ground, the tech pointed it out. It helped explain why, despite my best efforts to maintain my skis, their performance steadily declined through the season.
I bought a 7 degrees sidewall planer from the Race Place which has since allowed me to prevent that problem.
Some advocate using a round bit for cap skis because it removes less material per pass and the sidewall lip is generally thinner on a cap ski so you don't want to overdo things. If you're just careful not to get carried away, a regular panzar file in a planer works for cap skis as well as conventional sidewall skis.
When it comes to race skis, there are various schools of thought on round vs. flat files for sidewall prep and some other preperation techniques employed, as well. The Race Place link is posted below.www.ski-racing.com/