"What I don't get is WHEN I'd make any adjustments?"
The answer is after the preview but before the actual scan. There may be an "auto-fix" button that will attempt to make automatic adjustments to levels, curves, etc. and that may work just fine for your needs. I am not familiar with Nikon Scan (it has been way too many years since I stopped using it), but there *may* be a way to have it automatically apply its default optimization adjustments to each image it previews, or you may have to click on a software "button" after the preview in order to have the program apply its built-in best guess at optimization. You will have to read the manual to find out whether or not the software has a built-in optimizing option and whether or not it can be set to apply it to the images automatically (without your direct action).
Realize, however, that relying on a built-in automatic feature in the scanning software is exactly like relying on the built-in automatic exposure in a camera, or the built-in automatic adjustments in photo editing software. It may work well for some images, work OK for more images, and be downright wrong for some images. Only you can tell whether or not it will work for your needs.