Well, better machine shops like to have the pistons in hand before they bore your block. You are still dealing with the human element, and machining errors and/or collapsed skirts happen all the time from UPS drivers throwing piston boxes around all the time. It can just save you and your machinist a headache later if you bring them in advance. Also, just tell him the ring type you will be using and he should be able to take it from there.
