On PC monitors you generally had small amount of border, so that you would see all of the image without any lost pixels/overscan unlike TVs that always overscan and lose part of the image. Some montors may have poor convergence or focus near the edges so you may want to increase the border but in general it shouldn't be necessary.

Interesting thanks, I do have some minor geometry issues on mine, but it's a massive 21 inch monitor. I can afford to lose a bit downscaling it.

CRT screens aren't perfect rectangles - it's necessary to leave some blank space for the image to be rectangular.

The border isn't necessarily black, it can often be used as an extra pixel, of any color from the palette.
Many programs make use of the border for visual effects, good emulators and CRT->LCD converters should include the border.

The overscan border area definitely was visible on CRTs. Some games fill that in by either changing the first index of a palette or a register push. All of the Commander Keen games have a cyan overscan border.

In a Windows enviroment however, that border's off limits and is always black due to driver limitations. This would also prevent DOSBox from emulating the border color properly even with a CRT.

So when using Windows did you tend to scan it to the edge or leave a border..?

I run my corners almost all the way to the edge, leaving just a little bit of margin (couple mm or so) so that bezel won't cut anything off if I happen to change my viewing position.

Factory default was almost universally obscenely underscanned (as if to compensate for the ridiculous overscan on TVs? 😀 but more probably to make geometry imperfections less obvious), first thing you did after figuring out the controls was to make it fill the screen!

I always ran mine to the very limit of the viewable area of the crt, I would loose a few pixels/ lines especially in the corners but loosing a piece of the 95 start button was no loss since I knew what’s there, same on the title bar at the top, giving it a trim was well worth maxing the image size

Some screen modes would come up 2 inches from the left bezel, and you'd run out of horizontal position before you got it closer than about half an inch... so then you might have to leave half an inch all round if you cared about the aspect ratio for whatever you were doing.

Yup… on my main system the textmode is more to the left then most 320x200 modes… but not all! And because the horizontal and vertical frequency are the same, the monitor can’t distinguish between them. So I leave a border so I don’t have to adjust the position each time I switch between text and graphics modes.
And if I switch to another computer on the KVM everything is slightly different again… so I just gave up and accept a little border (so the overscan fills it up)

Yeah, KVM shifts image slightly to the left because the sync signals get slightly delayed by the switching mechanisms in them. It gets most noticable in higher resolutions.