It's like everything else. Will wrong geometry run, in most cases yes. In that vein will a motor run with single angle valve job and out of round cylinder bores, sure. The closer to perfect everything is, the better the motor will run. How far off can various specs be? That is the million dollar question. I've run motors both ways, sloppy and worn or as good as I can make it. It all comes down to how much time and money do you want to spend? Some items are worth a lot, some a little. To some it only matters that it stays together and runs. I say learn what is correct, make a decision if you feel that is important and go with it.
Doug