I am not talking about scaling.I run both 1366x768 and 1280x720 skins on my main HTPC which is 1024x768 (non-square pixels = 16:9) and cannot see any scaling problems.
The problem are the animations.
If you take blue2wide (which was designed for 1360x768), and run that with 1280x720, then all animations our out of place.
I.e. the basic home menue which "zooms" in from the center of the screen. on 720p, the startingpoint of the zoom animation is nolonger the center of the screen.
So if you are using such (zoom) animations in your skin, then the startingpoint of your animation will not be correct at any resolution, lower than what you designed it for.
The other way around is no problem at all.
As you can see, Blue3wide was designed for 1280x720, and it will work correctly on all larger resolutions without any issues in the animations.
Designers choose.
Austria