I lap them not to get them to seal, a good valve job should get them to seal, but you lap them as a poor mans way of checking to make sure they are touching all the way around and the sealing surface is more than just a sharp edge. Checking with fluid alone does not tell you if it has a good contact area, espercially important on the exhaust for heat transfer. Could also be sealing against the wrong side of the margin, I like to see them seal more against the outside edge of the valve face than the inside but just checking with fluid you will never know. Also if the machine shop assembles them they could have left a layer of oil or grease or something that seals them instead of the metal metal contact (I had a shop in texas do this one time and had to have them re-do a slant six head when I was 17 and they argued with me that they did not have to seal with just metal to metal, never used them again). If you have to grind them a bunch with lapping compound to get them to seal you can end up with a curved sealing surface, I have done this in a pinch on junk yard builds but it aint the right way for a good engine. Just use lapping for checking the machine shops work not grinding.