Joel
>THe hard part of scraping is learning to rub out a consistent layer of blue paint because the thicker the layer the less accurate is it's reading. The second skill - which more addresses your question - is learning to put the plane down on the surface plate consistently. when you have lots of inaccuracies wiggling is ok, but as you get closers you have to touch it down and pick it up without rocking etc. that will detect a convexity if you put it down the same way each time (which is a skill). However the point of forcing a concavity and then removing it is a good one - it's easier to do - but I havn't tried it that way. the first plane I scraped - which is still my favorite smoother after 20 years had a convexity from side to side and from front to back made by the dealer flattening with a belt sander - it was not fun to remove.
for the past 5 years I have used the selective sandpaper method I described above and the result is more than good enough.