Image processing: To select Sx in maketform of imtransform.

To apply imtransform, maketform uses [1 0 0; Sx 1 0; 0 0 1] for horizontal spatial transform. How I know image transformation is needed or not? What human eye see image is different from computer precision. How to select value of Sx for best horizontal transformation? If I calculate skewness, it gives same numeric value for left or right oriented image.