Duncan's 1960 Magnavox

[Electronic M]

As a general rule the proper setup is to have the screen just filled (maybe 1" of overscan) in both axes at the lowest line voltage you would run the set at. Some sets don't give width control so you get what you get or have to improvise.
Shango tends to give that vertical underscan advice but it's mostly chicken shit because 1.) no TV maker or repair shop would EVER do that back when these were in service, 2.) it doesn't allow proper geometry unless you can also adjust for matching horizontal underscan.
The one place vertical underscan is somewhat advisable in consumer TVs is on 60s color Zeniths...those Zeniths tend to short between secondaries from insufficient insulation and on those sets underscan lowers the risk. Someone is rebuilding those transformers now so it's not that big of a deal anymore.