I've pretty much concluded that if I have one whole map in landscape mode, and two half-maps in portrait mode, I'll have the bases covered about as well as they can be on current devices.
What I'm wondering now is what's the best dpi setting for the images to look good on a reader. 72? 96? I did my initial tests with 300, but that's overkill, and probably doesn't actually help the image to look any better on the screen.