Hi Steve
There doesn't seem to be any hard and fast rule. Whenever I see a railway or photos I'll always study the pointwork. It?seems to be on older or simpler railways both the blades and stock rails were planed with the main feature being the original rail feet are at the same level and some more modern lines have the blades planed so they fit on top of an unplaned stock rail foot as you suggest. This requires both long blades and very sophisticated planing of the sides and top of the blade rail, and sometimes special sections of rail. Have a look at Wenz Modelbau website for German examples of such blades.
I should add that planing both is probably easier - and the inner side of the blade should be planed first.
John
