>DOF should drive your composition with diffraction issues being a >secondary consideration.
I think diffraction effects are over-rated. I recently shot some mountain scenery with rockpools and wildflowers in the close foreground and mountains in the background. I needed to get it all sharp. f11 was not enough. Even with techniques like hyperfocal distance being used, too much of the image was not in focus.
f16 provided enough DoF and the images were way better, viewed in their intirety. The sharpest part has theoretically been degraded by diffraction but I can't really see this unless I zoom in to ridiculous levels at which the overall context of the shot is lost.
I would hate to see a situation where less experienced photographers read a thread such as this and afterwards feel inhibited about using apertures smaller than f8