I understand - oh so well - that it is the AIAG recommendation. And, I would say that for typical gaging ndc=5 may be fine. But, if you are trying to determine runs, other Western Electric rules - or any process activity - with a gage whose distinct categories indicate only 5 statistically valid "buckets", you have some pitiful resolution. I have seen SPC charts that jump in increments so large that they provide poor control limit analysis and overall poor control. It dilutes any predictive power you may have had. Point is, the people setting up the chart did not relate the statistical gage resolution to the resolution they thought they needed. Face it, they may have had 10 lines on their chart between the control limits, but with the gage they had, they only needed 5 lines. It would be a more accurate visual of their level of control.
That said, if (for example - with enough exaggeration to make a point) your control limits are 10% of your specification, your PV represents that 10% of the specification and you have a true normal (or similar mean-central distribution) then ndc=5 could well be tolerable. But, if your distribution needs to be controlled via adjustments for any reason (not just precision machining), then I would recommend buying a more suitable gaging system to get statistically valid 10:1 within the control limits. You need some decent resolution to let "the voice of the process" (to use a Dr. Wheeler term) speak in clear terms - instead of mumbling.
I am also aware of the concern for the calculation for the GRR that Dr. Wheeler has brought up (I have read both articles). What I have not seen is a corrected ndc calculation from Dr. Wheeler from that analysis. Maybe I missed it. That could be a good improvement on the analysis. The concept of statistically significant resolution is critical to measurement system decisions. If there is an improvement, I am anxious to see it. Even so, I would go with a "corrected" (but still based on USL-LSL) ndc>=10 unless atribute (or near attribute) is "good enough".