OWD et al, with modern guns like BSS, 23's, etc. built of modern steels you are correct. Barrels should fit tightly against breech with no gap at the top.
Older guns it is good practise to see a very slight gap at the top of breech faces, also slight gap beneath flats at watertable as Cherry bomb describes. The hinge pin will inevitably wear and the barrels will then fit snugly at standing breech which is usually cut at 3-5 degree angle. Some may not want to hear it but wearing in or just plain wearing of older steel hook and hinge pin is a fact of life.