My two cents: I like a button hole on peak lapels but agree that the gentle curve of a shawl will look disturbed by a button hole which in all likelihood will not be used anyway. I dunno what Capt. Jeff is on about, having a loop on the back of a lapel that will be useless if there isn't a hole for the flower stem to pass through???
My reference point for a lot of these questions is the old movies shown on AMC! I remember seeing someone from the forties with a very broad shawl collar dinner jacket once and a prominent button hole that looked way too ostentatious for the implied subdued nature of black tie.