Flatten with a stone
In general, I use the "sandpaper on a flat surface" method to lap my handplane soles and chisel backs. However, even the coarsest grit of sandpaper can take an awful long time to flatten the back of a blade, if it's out-of-flat enough.
The corresponding disadvantage is that you end up with a lot of slurry -- the cast-off sharpening particles mixed with the lubricant (the water). But that's OK, if it gets the job done.
So, as I said, I picked up a 300 grit waterstone. Seems to work. Above is a snapshot of the back of my larger morticing chisel, polished to near-mirror smoothness -- at least, the important end.
The point (pun unintended) of sharpening a chisel (or handplane blade) to such smoothness is that it provides a sharper edge: If the two intersecting surfaces -- which meet to form the cutting edge -- are this shiny, this indicates that the microscopic cutting edge will be smooth, rather than jagged and rounded over.
A super-sharp blade then means that you can actually slice the wood fibres, rather than plowing through them and breaking them. This, in turn, leads to cleaner results.