Hi Mike,
That was one the items stressed in some the “how to books” that I’ve read. When I started I had no idea what that procedure was for, but it became apparent when I did the first planking and the “garboard plank” (I think that’s the right term) would not lie flush or follow the curve of the hull. I had not permantly fastened the last three planks. I Removed those planks and used my Dremel tool with a Router bit to cut an indentation into the keel (the rabbet?). After replacing the planks I’d removed, I made a paper template of the area the last plank had to fit in. After cutting, trimming, sanding, gluing, all the planks seemed to fit. After an application of a liberal amount of Modeling Paste and more sanding, most of my semi-skilled errors weren’t visible.