I would use the wad you feel gives the best pattern. I seriously doubt there's a line that can be drawn in the sand on this one.

I'm a pragmatic kind of guy. The European bores are designated to the nearest tenth of a millimeter. which is to say they are measuring to the nearest four thousanths. Some might think that's a little sloppy and if a bore turns out a lttle on the tight side, hey, look, it still measures 18.4 mm, so all's good.

I checked. My Perazzi is 18.4 mm (their measurement) and my two Belgian Brownings are 0.725-0.726 in (my measurement). The oldest of the Brownings is a '66, so it's anyone's guess as to whether or not they gave any consideration to wad types.