I have a mechanic friend
I have a mechanic friend ,Master Nissan mechanic, went through all their training when working for Nissan dealerships. Became a consultant to Nissan service departments Nationwide ,taking calls about mechanical issues dealerships were having trouble solving.
He works in an independent shop now, regularly fixes head gaskets on Subarus, and he says Subaru heads HAVE to be dead flat and true or a new head gasket will fail prematurely. Most Subaru heads need to be sent off to the machine shop and milled true. My mechanic friend says even if they're more than a thousand out of true you're taking a chance at that gasket will fail. So that's a lot of more expense right there to do these right.