Many years back when I first heard about people grinding their planes flat and sides perpendicular I check out all of mine and they� are all dead on. It just never seemed to make sense that anybody would manufacture a plane and not make it dead on. I have never run into that.

I recall stories about planes needing to have the bases re-ground, but never had a problem with any in my collection. One idea around was that the castings weren't being 'seasoned' long enough before machining, and were warping after manufacture. When I was a teenager there were stories about yards full of stacked engine block and head castings at the auto plants, waiting to 'age' before machining, but those may have been urban legends.

Unfortnately, the fad for grinding the bases of planes means that if you buy an eBay plane (or sometimes even find a used one in an auction), there's a good chance that the base has been thinned enough to weaken the mouth/throat area. You can often spot cracks there.

I have flattened the base on a few home-built wood body planes (remember the James Krenov craze?? ;-) ) . More research later revealed that Japanese plane craftsmen had a very involved scheme to scrape hollows in the bases of their planes to improve performance!

One thing I learned from guitar builders was to 'knock off' the sharp edge between the sides and base of the plane - it avoids gouging adjacent surfaces when working in corners. Most smaller planes used for shaping instrument bracing have a soft radius there - assuming the casting is 'meaty' enough to allow rounding the edge.