The main concern is in situations where the plans call out the use of washers as spacers......
Even though both washer designations are aircraft grade hardware, the NAS part has a much tighter tolerance for thickness than the AN washers have.
This causes a lot of problems when calling out a specific # of washers for an assembly that they are used as spacers in.
So in instances where you are just using a washer for the common purpose of load distribution and abrasion protection AN960's are fine, but NAS should be used when it looks like a specified stack-up is dimensionally sensitive.
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Van's Aircraft Engineering Prototype Shop Manager
RV-6A (aka "Junkyard Special ")