Defaulting Business Account Class for BAs created from customer

How can you default the Business Account Class for BA’s that are created automatically when you create a new customer?  We have the default setting from customer  management preferences and that works great for BA’s that we create as a business account and then extend to a customer.  But…  if we create a customer, the automatically created BA does not default a business account class.  I couldn’t find any AR preferences for the Business Account class.