50% gray is the way to go. I've been doing it since we had to simulate gray by dithering black and white pixels.

I agree. Black and white often pop up against certain types of content (e.g. white spreadsheets on black background, dark mode terminals on white), but gray's a good compromise.

black wallpaper will be awesome for battery life once OLED laptop screens come out

It's not really true that black wallpaper saves battery. Black wallpapers have been shown to make almost no difference on smartphones.

I mean I guess if your primary use of your laptop is sitting on the desktop with no windows open, just staring at it?