The idea is to start fresh, whether you do it before or after doesn't matter too much.
A lot of people who have problems haven't done a factory reset, same thing with the ics update. I started fresh with both updates and I have never had a single issue. Sometimes old files or stuff doesn't work right with new updates and it causes problems. That way if you do a factory reset you are fresh with no programs or files to mess anything up.
Think of it like a computer, you wouldn't do a complete os upgrade without starting flash right? Same thing goes with our phones, they are like mini pcs, that's what a lot of people don't understand