I too was trained by the IA back in the mid 60s and also was not taught about any need to frame up or down other then to keep it in frame. Of course some theatres didn't have their masking set at the proper aspect ratio and didn't show the entire vertical image, and thus one would need to center the frame as not to crop everything only on the top or bottom.
However, with the additional soundtrack information that has been added to the edge of prints over the past decade, it became necessary to slightly shrink the image area of each frame to make room for that additional information. You have noticed that on many flat films that the frame line is very wide while on others it's still the normal thin line. The ones with the wide line have been adjusted to only show the image in the proper aspect ratio. The others with the thin frame line are showing more image then what is meant to be seen, and thus needs to be propering framed vertically.
[This message has been edited by RoxyVaudeville (edited November 08, 2007).]