I think there are a lot of places using many different types and blends of cheese and in may not just be regional. In our area of upstate NY, there are some who use Mozz, some use strictly Prov, and some that use strictly American. Then there are the ones that use a Mozz, Prov, blend. Another popular blend in Mozz, Prov, Cheddar. There are 2 big ones in our area that use a Mozz, American Blend. And then There are those that use Mozz, Parmesan or Mozz, Romano. It all depends on the flavor that goes well with your sauce and crust. Every blend won’t go well with your pie. Do some experimenting but remember… the people who buy your pizza now, like what you use now, and may not like a switch. If you do decide to switch, my advice is to do it gradually so that your current customers don’t notice a drastic change.