Hi Fred,
Certainly the soundstage will shift upwards, but it's the midrange driver that carries much of the important musical and audio content, not the tweeter. The tweeter largely reproduces harmonics and overtones, except for the upper octaves of the piano, violin and other instruments as well as percussion.
Everyone thinks that tweeters beam like a flashlight. Modern titanium dome tweeters and aluminum drivers have very good off-axis dispersion so you don't have to adhere to the "exactly-at-ear-level" rule. There is considerable flexibility. Besides, with surround sound home theater, it's the center channel location that's really critical. You want it as close as possible to the video display so the image and dialog/on-screen sounds are one.
Alan
