Digital BW, The Print

Perhaps some of you can help with a PS problem that I've been 
trying to solve.

I prefer to regularly feather corners/edges on practically all of my 
photos. I've been selecting with either a rectangular or oval 
selection, inverse, apply a 250 feather and repeat with another 
100-150 feather and finally a curves adjustment layer. 

This works fine most of the time but whenever I have open, 
evenly illuminated areas, i.e. blank sky, calm water, I get a slight 
to horrendous posterization. It's very subtle on the monitor but 
evident on the output. I've tried combining two different selections 
to acheive a blended feather but that's not competely effective 
either. I've tried a burn/dodge layer mask/soft light/50% but the 
999 pixel diameter brush just isn't big enough resulting in 
uneven burns. 

I'm scanning at 16 bit then to 8 bit and 70-90MB files.

Any other ideas??

Thanks - R Murai