The "visible scan lines" on most emulators are overblown, and in general I keep effects like that turned *off*.
First off old monitors, especially color monitors, tended to have blurry enough focus that the electron gun would make a glowing spot somewhat larger then 1/525th of the height of the screen. Which means that even though the ][ used the NTSC screen "progressively" instead of interlaced it didn't leave perfectly black blank bars amounting to half the screen area. Secondly, with the exception of the IIgs almost everyone used composite monitors on their ][s. Composite signals are far from being "digitally perfect", and the noise and jitter thus encountered would tend to blur the screen even more. (*Any* vertical jitter between frames has the effect of further minimizing black non-scan lines.) And of course, from any distance away from the monitor even if there *were* faint black bars, your eyes would tend to simply screen them out. (Your eye is attracted to bright things, not dark areas.)
That said, it's true that, yes, if you look at an old computer's screen up close enough you can see spaces between the scan lines. IBM's CGA RGB video standard was particularly "bad" because it was actually "too good"... the monitors were *very sharp* and used a digital interface, so there was no compensating blur. A *pin sharp* monochrome monitor will show similar lines on a ][. However, a ][ connected to a good color monitor will have much less pronounced lines, and one connected to a period TV probably won't show lines at all. (Or at least, lines any more visible then those experienced watching TV. Large CRT TVs often have visible scan lines *all the time* simply due to their coarse shadow masks and the sheer size of the pixels.)
Anyway, it's a matter of taste. I personally think trying to emulate them is a distraction, and in most cases the emulation just isn't very authentic. To get them right you have to grossly oversample the screen resolution, add some *random* noise, and to *really* do it right you'd also want to emulate the spherical distortion of a curved picture tube. (Even if you're still using a CRT monitor it's probably a lot flatter than the ones in circulation in 1980. I've seen some attempts at that, but usually they suck.) I'd say if you really must have a "realistic" experience plug your TV into your computer, run the emulator on that, and let real analog circuitry do the mangling.
