Here's what the Green Manual says about checking the valve stem height after valve and seat work.

If you take too much material off either the valve face or the valve seat, the valve stem tip will "stick up" higher in the head. Too high, and there will not be enough clearance to get the lash clearance within specs.

The special tool, 9995222, is available to make the valve stem height check very easy.

The second illustration shows the minimum distance allowable from the 'keeper' top to the tip of the valve stem. The reason for that minimum distance is to prevent any possibility of the lifter bucket from coming into contact with the upper retainer.

If THAT were to happen, the upper retainer could be made to move downward, possibly allowing one or both of the 'keepers' or 'locks' to become dislodged, and possibly fall out. If THAT were to happen, the valve would drop into the cylinder: resulting in a catastrophic failure.

"Dropping a valve" means instant engine damage....SEVERE engine damage.