In general woodworking, anytime you veneer one side of a piece of wood you must also veneer the other side, even if it is with some other kind of wood. The thinking on this is to keep the one side from moving too much so that it warps the wood substructure.

I have not seen this to be an issue with guitars though. I think the reason is because the headstock is such a small area that it really doesnt matter and the veneer is so thin. This is the first I have heard of it in the luthier world.

Don't overlook the fact that archtops (a la Benedetto) have truss rod access through the headstock. This further reduces the already vulnerable neck-to-headstock short-grain transition. Take another look at Benedetto's illustrations and draw (yep, a pun) your own conclusion. 

Hard to imagine NOT using a reinforcing backplate on headstocks with truss rod access, no matter what the wood choice.

Now, if you plan on soundhole access....

In the end, I am of the school that believes that a stiffer headstock transmits string energy down the neck better than a less stiff one. Which is a good thing. I use backplates on my steel string guitars.