Hi m_ski,
I agree that when resizing, "aspect ratio" changes can make buttons look a bit funky.
I am under the impression that most installers, and "casual skinners" don't have the resources to make (or have made) buttons, and mostly just need to make buttons a bit larger or smaller without "ruining" the look of the buttons. Maybe (IMO) a "Preserve Aspect Ratio Resize" and/or a "Preserve Aspect Ratio" option box in "Properties/General" would be easier to implement, and would still solve the problem you (and I'm sure others) are having.
The work-around I use is as follows:
I re-created almost all the buttons in a "square" format (I used 100x100, 200x200, 300x300, and 400x400).
For "square" buttons I maintain "width=height" and resize at will.
Where I need rectangles, I use a pre-defined "width:height" ratio. (Anywhere between 4:1 and 7:1 works well for my skin since my most popular button sizes are 175x35 and 210x35)
Where I needed a "special" size, I just made it or had it made. If you're interested, email me, and I'll send you some examples.
Hope that helps.