I wonder when the thing about flattening the backs of plane blades and 
chisels arose?  Is it really necessary? does it make a bog difference? 
I've never flattened mine.  I'm pretty sure the old masters didn't do 
it, but I've never met one so I wouldn't have been able to  ask.  Thing 
is if they did it, then it'd already have been done.

Belt sanders can't produce a very flat surface.

I gotta say,  pushing  4.25" of steel  plane blade across anything  
sounds like  work.