Campagnolo states this in there instructions, (in the 80's and 90's at least, not earlier) With the mass and cables that usually go along with the stem, I use a empty stem and insert it the same amount of the same type and depth, wedge or conical expander. Adjust headset, remove and then insert the functional stem with bars. It is easier to tell if you have the headset assembled with the least meaningful amount of mass to "feel" the bearings.