Horizontal overscan on NTSC
one family of computers that was highly regarded for its
  graphics capabilities in the age of NTSC, considered 192 to be into
  overscan, and 160 color clocks per scan line to be the normal safe
  value.


That sounds about right. 

To get a consensus you could look at other computers which had NTSC output. For example the Commodore Amiga had a 'low-res' (320x200) pixel clock width of 140ns, which equates to 160 NTSC color clocks. The Amiga could also do 'over-scan' up to 376x242, which was intended to cover the entire visible screen area and beyond. This equates to 188 color clocks. 

Some other computers were limited more by the design of their graphics subsystem than the amount of overscan available. The Mattel Aquarius produced 140ns wide pixels and 320x200 resolution like the Amiga, but its crude graphics chip couldn't accurately center the image on screen so they limited the horizontal resolution to 304 pixels (38 characters per line) in software.