"He who marches out of step hears another drum." ~ Ken Kesey
You have given us your mechanical reasons for printing it your way.I would ask them for the mechanical reasons why they want to print it their way. There could be several reasons why "their way" works the best in your shop currently. Anything from ink selection/ink opacities/current dot gain/in house wash testing/customers wash testing/etc...I see both options as being viable for different reasons.I personally like to have a flash, like others have said, to keep the boards warm and the glue sticky. Assuming waterbase tack of course.Maybe you can add your black screen at the end of the run to show them what it would be like?
"Keep the boards sticky" I get, but even in a New England weather it's never below 85° in our shop, or so it seems anyway...