I'm always interested in the history of food and the traditions behind it. However, many foods such as pizza, ramen, and many others, have traditions that go back centuries and have split countless times into regional and cultural variations. What I find painfully annoying is when someone claims that the one version they like is the only "traditional" one and all the rest are crap. Actually, no, what's even more annoying is when that person is exploring cuisines that are prepared in some way they aren't familiar with and say condescendingly that it's nice that there's this OTHER way food can be prepared, but what they really like is THE ONE AND ONLY "TRADITIONAL" way, which is inevitably the way they are most familiar with.
Well, here's the thing. If what my Saigonese phlebotomist told me is true, and she had never had the peanut sauce variation with her goi cuon before coming to Portland, than it would follow that goi cuon w/ peanut sauce may be a Portland "tradition" of sorts (here we start to get into the wobbly land of how far back you have to go before something is a bonafide "tradition", i.e. a "long-standing", "customary" practice).
I think it's fair to say that the peanut sauce variation is authentically Portland Vietnamese, because that's how it's done here. Whether it's "traditional"-- I wouldn't go that far.
But that does not change the fact that the people who invented the stuff, namely the Vietnamese, eat it with the fish sauce mixture. Now I don't know how far back the fish sauce tradition goes, but I'd venture to say that it's likely a whole lot longer than Portlanders' tradition of eating them with peanut sauce. That's not based in any sound facts-- it's just my hypothesis, based on my experience with the stuff.
None of this speaks to personal taste, which is entirely subjective, of course, and is a completely different discussion. Although people are compelled to conflate the two arguments, it does not logically follow; they are entirely separate discussions. Personally I find the flavors in salad rolls very light (nem nuong versions notwithstanding) and the peanut sauce dominates the flavor so much that the rolls taste like crunchy, salady one note peanut butter. The fish sauce is light, so doesn't cling the way the thick sauce does, and adds all these marvelous layers of sour, sweet, spicy, salty and umami. it's like a wonderfully light but powerfully flavorful salad dressing- a perfect complement to the lightness and crispness of the salad rolls. You may not agree, for whatever reason, and that's your right.
In this specific case, I personally feel that the VN got it right. Give me fish sauce every time, and save the peanut sauce for the jelly and bread. (or satay, whose bold meaty, charred flavors can stand up to the peanut sauce without being dominated by it).