Currently i am using this
if(ratio<1.4) goto fullscreen;
else run in windowed mode;
Just so that those 16:9 screens wont get odd graphics.
Or ... just write your GUI properly so that it uses a layout manager and doesnt arbitrarily stretch the screen? [/facetious]
Seriously, though, you should always use a layout manager, even if its just a very simple one that adds borders to the sides or top/bottom and keeps fixed ratios internally. IME its pretty easy to write your own manager that will cope with *any* resolution, and then you dont annoy the people whose large monitors you're wasting