On the Model 45 manual it actually states: "The cutter is correctly set when it LIGHTLY touches the bottom blade across its FULL length."
That would suggest to me that having them set not touching is incorrect. Try cutting with a pair of scissors that don't touch, they don't work at all. The way cylinder blades pass across the bedknife is the same as scissors so it makes sense to have light contact in order to cut properly. If the blades and bedknife are lapped correctly, there shouldn't be any real problems with blunting the cutting edge, because they would line up uniformly, much like a pair of scissors do.
