Flashing paper whether that is by pre or post will work wonders in some difficult printing situations. However some people use it as the method of choice in each and every situation and that, I think, is unfortunate.
I think that the reason that some use the method indiscriminantly is that they do not know of another method to reduce contrast.
Let's be honest here. The affect of flashing is proportional to highlight values in a print. The compression of tonal scale is far greater in the highlight values then in any other portion of the tonal scale. That sometimes is appropriate and sometimes is highly inappropriate.
The reason for reduction in tonal scale is to allow higher local contrast to exist within the limitations of overall contrast characteristics of the printing material. Most people are capable of visually responding to greater local contrast in the mid tones and the high values. So in effect flashing produces a reduction in local contrast in one of the regions of desired greatest local contrast.
So is there another method that will work? Yes, there is. Contrast reduction masking of the camera negative whether that is through the use of low density and low contrast unsharp masking or through some other type of contrast reduction masking does work on the other end of the tonal scale. The compression in tonal scale, using masking, occurs in the low tonal values. The effects are predictable and repeatable. The effects are also widely variable depending on the type of mask one produces.
No specialized registration equipment is required for the process. A bit more work, undoubtedly. But for those of us who want to produce the best that we can, then the results are worthwhile and the efforts required not unwarranted.
I post this as information only. I encourage those who aspire to be fine printers to learn and to use the appropriate method for the requirement.