When I modified one of my bikes recently from an Octalink HT1 crank to an external bearing HT2 crank I first installed the external bearings snugly and did a try-fit with the new crank's spindle. The fact that the spindle slipped through both bearings with no binding or force indicated the bb shell faces were adequately parallel. If the spindle had been a force fit, I'd have had the shell faced.