Using cornstrach vs powder sugar is like butter vs crisco or scratch vs mix. What works for some doesn't always work the same for others. I say do what works best for you. If powder sugar works for you then don't change that just to do cornstrach cause others say it is better.
Now I like rolling mine out on crisco, so you can pick the mat up and flip it all on the cake with out having to worry about sticking my nails in it.
And so you aren't having to pick up the fondant and move it out to make sure it isn't sticking.
I do use powder sugar if I am rolling small amounts out for cut outs and things, but for covering cakes it is crisco.