I'm not sure I've even seen the plugs on mine. I think they're due for preventive maintenance replacement sometime after 100,000 miles.
I'd get a second opinion, there's no "tune up" listed in the maintenance schedule I can find and at $90/hour for labor that's a significant labor cost. Maybe I'm still living in the past, but twenty bucks per spark plug (part) seems expensive too!
When I bought my used 07 last summer it had 22K on it I spoke with one of the Ford mechanics at the dealer and he stated I should have the plugs changed before 30K miles because they will become very hard to remove as it gets old/mileage.
I have not done it yet as I have only put 5K miles on it since last April. just an FYI for the rest of you.