Marcus,
That's a great reply and the reason I think so is this: I've worked at places that sell springs to retail customers and every-so-often you get the clown who buys a spring and then takes it to another shop to get it "rated". For example, you sell the guy a 5.6kg spring. Then a week later he calls and says "hey, the spring you sold me isn't really what it's supposed to be! So-and-so put it on their spring tester and said it was a 5."whatever"! I want my money back!".
Well, number one - there is a tolerance of +/- 10% on all springs. And, I'll believe what the manufacturer of the spring tells me it is over "so-and-so's" spring tester any day.
Number two: Who's to say "so-and-so's" spring tester is correct in the first place? Yeah, right - the answer is always, "we have our tester certified three times a month". Sure you do..........
Number three: Spring testers are usually a joke anyway. No two will ever give you the same reading, so why would it read what the manufacturer of the spring says it is?
Bottom line: If you buy a spring made by a reputable (operative word) spring maker, you should be able to trust what the marking on the spring says it is. The spring is designed (by wire diameter, number of coils, etc.) to be a certain rate and if you figure in production tolerances, it usually is within those boundaries.
Second bottom line: If the spring gives you the sag numbers you're looking for, what else matters?