Is the vicar coming to tea?

I poured some juice into glasses for us this evening, and because they were only about 80% full, Jeanne asked me this rhetorical question. I got the allusion immediately - my father would have said "You'd sell more if you filled 'em" - but I had never heard it before. Her explanation was that the space between the liquid and the rim of the glass forms a circular band resembling a vicar's dog collar. Is this a well-known phrase and explanation?

wrmst rgrds
Robin Bignall
Quiet part of Hertfordshire
England