Digital BW, The Print

Aaargh-

I've been having problems with posterization introduced into a PS7 
file, that is very slightly visible in the print.

The image has very similar tones around each of the four edges of a 
greyscale image.  To help with the center of focus I want to darkening 
the edges.  To do this I make a rectangular selection, invert the 
selection and then feather it by 50 pixels.  When a curve is applied to 
the selection the feathered edge has 'bands' in it.  It is not a smooth 
looking transition but steps in concentric bands from tone to tone -- 
I'm calling this posterization.  It is not that obvious but once I know 
it is really distracting.

Adding a small amount of noise to the image mostly removes the 
posterization but (and it may just be my imagination) leaves a less 
smooth texture to the image.

So, is there a better way to make the selection without introducing the 
banding/posterization?  Is it better to go around the edge with a big 
soft brush and paint it in the selection?  Is there a setting somewhere 
that will introduce some dithering or noise in the feathered area?

I tried making the curves on a 16 bit image after loading the 
selection(s) from the adjustment layers of an 8 bit version but got the 
same results for the most part.

Any suggestions are appreciated.

--
Kevin