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FWIW, I have opinions and information on several things in this thread:
The scanning of the corners of the image onto the bell of the round color tubes was detrimental in another way besides content loss - the 21kv electrons would generate about 16 kv or so secondary electrons, which would scatter all over the tube face and light up all the phosphors a bit, reducing the contrast and color purity whenever there was somethng bright in the corner of the image. With the inefficient early red phosphor, which required more beam current, this meant that the green and blue lit up more with this stray light, so a pure red was harder to obtain in practice. Even though the red phosphor itself may have been a bit more saturated, depending on picture content, the blue and green were lit up somewhat when they shouldn't have been. Later picture tubes did much better at maintaining color purity and contrast into the lowlights when the picture also contained bright areas, and this was just one reason why.
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As to seeing things in the corners and edges - just play an old color show, like the last Howdy Doody show, onto a 16x9 HD set (so the left and right edges are completely visible), and you'll see plenty - plenty of misregistration, target flicker, horizontal sweep ringing, and other TK-41 camera shortcomings.
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Biggest problem with cutoff material on roundies in the old days was movie titles. these days, calbe box menus, DVD menus, etc. all extend into the roundie cutoff corners.
On the other hand, sets were too wildly variable in centering and scan size for anyone to put an Easter egg in the corner and be sure it wouldn't be seen.
The edge cut off might be anywhere from zero to 10% of the picture. Solid-state sets got more stable, but not always any better in manufacturing tolerance. A decent solid state CRT set with adjustments (like CRT projos) should not overscan more than, say, 3%, in my opinion.
As time goes by, it will be less likely that flat panel sets will need overscan; but at this point there is still a need for some, since the source material in some cases has suffered a bit of loss while in the analog domain. People are gettting used to 25% black bars when the aspect ratio mismatches the screen, but just a little black that comes and goes with source material looks like an annoying malfunction.
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Regarding old professional monitor underscan, professional color monitors always had an underscan switch. This was essential in seeing the whole image while setting up a camera for registration and proper scan on a (printed) test pattern. These monitors also had a "pulse-cross" switch that would offset and enlarge the vertical scan so you could see the vertical sync pulses and determine quickly if they were normal.
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