Mike, that is most often the case, but it's either the opinions/potentially skewed accounts of those who were there versus nobody's. Guralnick and any other author relies on those same people, to varying degrees. But I think it is of great interest to hear those accounts, which can be beneficial.
First of all, even without filtering out the BS and determining who has what agenda, etc., it's like there is some cumulative truth once you lump all those stories together. Even though you get some erroneous information, you end up with basically the truth. You might not know exactly which elements ring the truest, but you have heard all the accounts, so the real story probably can be told by borrowing parts from each.
Secondly, even though you do have the truth and then some, you do need to find a way to whittle down everything you've gleaned into something which might not be totally specific, but is something you can hang your hat on (or else you'll go crazy

). So you read all this stuff and come to learn what most of these people's motives are (or simply how bad their powers of recollection have become), based on what you can determine on your own, and on what the other central figures have to say about them...and of course what
their motives are, and so on.
Having heard certain accounts before, you can also get a step closer to the truth by noting the elements of them which certain people
don't talk about as well.
I don't know if I'm articulating this very well, but mainly I'm saying it's educational simply to take all this information in and decide for yourself what the truth likely is. You might not always be right, but you can't ask Elvis, so that's about all you are left to do.