Custom GUI.Button background texture

I suppose default Unity's button texture on background has some extra coordinates or settings, when I change button's height and width it's perfectly stretched. Can't say it about my controls.

I know how to slice images for XHTML/CSS and I used to design skin for windows application, back then developer game me some utility to slice it right, where I had to set coordinates for corners and middle part.

Anyways, what's the best way to work with custom round cornered buttons? Should I drop this idea and use default ones? Or just design buttons without rounded corners?

Thanks.