I usually try to get it right during capture. I usually do a 5 minute capture and see if the black level appears to be right. Usually on a Japanese disc the black level will be between 0-16 so there's no need to mess with anything because that can be adjusted in post no problem. It's more important imo to pay attention to the white level. The white level on the JSC for example extends actually a little bit above 255 and the only way to capture this without it getting chopped off is to extend the luma white point in virtualdub during capture. One of my players chops off everything at 255 but luckily it has d-ext so I can turn that on and recover that information if I use that player for capturing.

Btw, the black level on the JSC is around 0(for the black bars) but the active picture area's black level is a little above 16. I believe this was a goof-up on their part and is the reason why the white levels extend past 255. Here is a little example. I had the wrong capture settings here and that's why the black level for the bars appears over 16 instead of down around 0 like it should be but you get the point.