How Marty McFly Met Doc Brown

Really who cares? All this back story stuff is a help to the writers in order to make relation ships work and often to maintain continuity. It shouldn't matter to viewers at all. It certainly shouldn't make any difference to the way you respond to the movie. Sad.

So Marty wasn't just being friendly with Doc Brown so that he could use his amplifiers? (opening scene of the first movie).

Wow, I'm even geekier than Bob Gale himself about Back To The Future. It turns out that I printed the scripts of the movies several years ago and there it's written the way they meet each other (there are many changes between the script and the final movie, but those shouldn't affect the way Doc and Marty come across), and it's not the same explanation that Bob gives in the provided link (and I quote de script):

MARTY
Doc Brown's all right -- he's just a little hung up on time. A couple of years ago, he showed up at mu house and hired me to swwep out this garage of his. He pays me 50 bucks a week, gives me free beer...and gives me total access to his record collection -he's got this great old record collection.

http://www.imsdb.com/scripts/Back-to-the-Future.pdf

So they meet like that :)

Greetings from Spain, Jorge.

P.S: obviously you can check out what I said in the link.