Ignorance is a wonderful thing, saves me fretting too much over these, as I miss most of them. However, when I DO notice (altos on screen, tenors or baritones, sax that is, on soundtrack; full orch. on soundtrack, small group on screen) I DO get annoyed.
I love that scene in Blazing Saddles when the cowboys gallop by to the off-screen music of Count Basie, come round a corner and there's the full CB Orch. playing in the desert.
I enjoy corny musicals but always annoy Herself by banging on about the fact that the numbers you see them rehearse throughout the film are never the ones they play when the show opens. Not to mention the Busby Berkeley dance routines that no-one in the audience could see except the man on the overhead camera!
RtS