I'm for edge blocking if you have no other choice. This is strongest part of sword and can really block even strong cut without too much damage. Better to damage a blade then one's own head, isn't it? Flat side will bend or brake very easy and one gets cut/kill.
how do you know the flat will bend or break easily?
it is true that the blade edge is stronger harder steel than the back or upper half of the flat, but it's not simply a matter of which is stronger
it's a lesson in physics: a larger surface area will have more space to absorb a blow. A narrower surface area against an equally narrow surface blade concentrates so much force against that area, and though the edge is the "hardest" part, it will break before it bends, resulting in potential for catastrophe for your sword (mutiple small nicks on the blade edge will hinder your sword's cutting ability, an edge to edge blow in the same place over and over will increase the risk of your sword being rendered useless (the other blade chops through your blade end into the softer steel, and if it's bad enough you have to reforge your blade) or even shorn completely off!). Whereas the flat has both hard of edge and softer steel (which is flexible, and therefore can absorb such an impact with greater ability). If you don't believe me, then get a couple of knives (or swords if you have them, but preferable something slightly expendable) and bang them togethe forcibly edge on edge. Then take another knife, and bang it against the flat of another knife. See which one is more damaged. (former should have little nicks, latter should have almost no damage to either blade). I know knives aren't the same as swords, but it's the same concept.
And I do realise that given the alternative (i.e. death), defending with the edge is better than not defending at all. better your blade than you, so you're right about that part.
oh, and here's a link for an article about edge on edge, and though it's with western swords so might be a little confusing, it's close enough:
edge on edge realities