Hi Elwood, 
I have had the same experience as Joe, since the tool tips slightly as it is moved across the key bed.  Using a straight edge like the one Joe sells works very well.  Please visit my website at www.key-leveling.com to see a product that really speeds up key leveling.  Watch for it at the upcoming convention, and in the July PTG journal.
Sincerely, 
Christopher R. Rawson, C.P.T., R.P.T.
