Zero washers, 1 washer, 2 washers.......all acceptable in my book with a 3000 or higher stall converter. I've had 2 in my Impala ever since installing the original Yank 3000 in it, and part throttle shifts have always been very firm but not harsh.
But here's my experience that better relates to the OP's situation:
Transgo kit + 093 servo (OE in ImpalaSS) in my pickup: Zero washers with stock converter and you would definitely not want the part throttle 1-2 shift to hit any harder. I'm still running 0 washers in it now with the SS3600 converter, and I consider the part-throttle 1-2 shift to be as near perfect as you can get. Very firm, quick, but nothing approaching harsh.