From my point of view, adding something, ie; a block of wood/steel between the jack and a axle is could lead to a failure of the jack to have a good "purchase" of the axle housing.
Of course, many of the original Ford axle jacks of the '30 had an attachment that flipped up for use on the rear axle.
Using a jack on a frame is a whole different matter.. A block of wood between the jack head and the frame would be a good idea to minimize the likely-hood of bending the thin frame metal..