500 EFI upper rebuild?

Thanks for all of the responses to my questions in the other threads. Now, the question for the day; when should the 500 EFI have an overhaul -- and what should be done (complete? Valves/springs?)

The previous owner took great care of her. The motor has pretty much been babied, certainly not ran hard. I have about 375 hours. No problems. I've been told that @400hours mercruiser recommends service overhaul. I probably won't do anything this season, but can do what's needed at seasons end. I'll probably end the year with about 450 hours

Nothing in their manual on the 500efi about rebuild.

http://www.mercuryracing.com/_media/...5021%20601.pdf

Efi not so much, carb is a must. I'd leak them down and see where you stand. 10% differential and I'd be looking at refreshing the motors