I have always been frustrated when I stretch a Curved-corner Box that I distort the corners.
I have just discovered it is my own fault for shunning the Smart transform option as with it set, the corners keep their aspect ratio.

Now knowing this my personal workflow does not require ST to be on at all, I just need to hold down the Alt key and stretch to my heart's content.

@Xara, why you block off the second Curvature box defeats me as once you "distort" a box, you cannot recover the corners in any easy fashion.

Doh!
Acorn