When I first started using Neat Image, I was confused about this also. I didn't understand that what I was seeing in my processed images was an incorrect noise level setting, rather than something to do with the artifact removal. I find the Frequency Components display arrangement that shows the image, along with the luminance (or Cr or Cb) information at High, Mid and Low frequencies, to be extremely useful for adjusting the Noise Level setting.
With the frequency display arrangement, I set the Luminance Noise Reduction Amount to 100% (I know, I could use the Filter Tuning Mode, but it's just as easy to set Luminance to 100%). I then adjust the Noise Level values, starting with High only, until I don't see any of those linear artifacts in the corresponding frequency display. I look through the image at a number of places, usually at 100% or 200% in order to see the small artifacts. When I like my High results, I bring up the Mid Reduction Amount and do the same and then once again the same with the Low. The Very Low and Ultra Low are best evaluated by direct observation of the full image. I don't need them a lot anyway.
It is sometimes necessary to adjust the overall Noise Level and/or the Luminance Noise Level in addition to the individual frequencies, but most of the time, the individual sliders provide enough adjustment. I usually find myself pushing the sliders to the right, but I have found at times that a negative adjustment of the Noise Level can be helpful, especially at lower frequencies where blurring of the image detail becomes an issue.
Once I have the filter tuned up, I simply back off of the Luminance Noise Reduction Amount a bit. Since I use the Photoshop plug-in, I usually lean a bit to the overprocessed side, since I can fade the effect of the filter in Photoshop if it looks like I have overdone the effect.
Prior to discovering the Frequency Components display mode, I felt lost trying to adjust the Noise Level sliders. Now I feel as though I really have control over these settings and it is rare that I see the little dashes appearing in my processed images anymore.