Steve Elliott
Some of my work with chisels requires a very flat back, more than just 1/2" or so because the chisel back is the guiding surface. For these chisels I carefully flatten the back when I first prep them and then only touch the back to a very flat finishing stone.

For any blade with a secondary bevel on the back (plane blades, for instance) and for many chisels the concavity of the long axis of a stone would hardly matter.