Graham, I've researched extensively regarding toxicity of nitrite, once I read, for the first time, that it was more toxic than ammonia (I was very skeptical). At no time did I ever come across anything that suggested the opposite. Since I've also researched this again online, very recently (sometime last year) perhaps evidence is even more conclusive now about nitrite being more toxic than ammonia than it had been a few years ago. However, when I first started keeping fish, I remember that the popular "fact" back then was that ammonia was king of toxicity. That being said, like you stated, 6 of one, half dozen of the other - both will stress and eventually kill fish either directly or through secondary means.
Regarding the use of salt for prevention of nitrite poisoning, I've also read different opinions on dosage, including one article that said only a very miniscule amount is actually necessary to prevent nitrite poisoning - not even close to the 1 tsp. per 10 gals. that I have had to use. I personally would never use 1 to 3 tsp. per gallon in a Discus tank however... JMO. Last year I lost my bio filter when I opened my new tank. I used Prime and salt. Since Prime detoxifies nitrite but only for a set amount of time per dosage, I used the salt as insurance along with it.
And yes, you are right I learned about ammonia, nitrite and nitrate (back then nitrate was considered TOTALLY harmless unless is rose to incredible amounts in your tank - hmmm... probably why I kept losing my Blue Rams - and it was said that WCs only needed to be done once a month!) and their affects on fish the hard way when I first started keeping fish back in '91. Set up the tank, bought a whole mess of fish, plopped them in and wondered why they were dying after a week. Too bad the LFS didn't bother telling me about the bio filter and cycling a tank until I went back to them with questions!