Upscaling 720p to 1080p vs upscaling 768p to 1080p Suppose I can run some graphics in 1366x768 equally well as 1280x720, and I have a 1920x1080 monitor.

There are more pixels in 1366x768 than in 1280x720.  However, in 1280x720, a 2x2 grid of pixels fits exactly within a 3x3 grid of physical pixels in my monitor.

So which should look better, and is there any theoretical basis for which one looks better?  Does the 3:2 ratio mean anything at all?
With a good-quality upscaling filter (bicubic, for example), there's no particular importance to the 3:2 ratio. Starting from a higher-resolution source will produce a better-looking final image.

It's true that if you're programming the upscaler from scratch, making one for a fixed ratio like 3:2 is much simpler than making one that handles arbitrary ratios. But image-processing software, monitors, etc probably have arbitrary-ratio upscaling built in anyway.