I completely agree with you on this advoce. To anyone, even experienced users, ALWAYS back up your reg files before making any changes. It is a simple enough thing to do, and it will save you alot of heartache in the long run.
As far as recovering file assc., the only suggestion I would have is to perform a "restore to last known good" boot option.
Combing thru a reg is like trying to untangle wet spaghetti, you get one piece straight, then something else sticks together. I must admit, I always keep a fresh back up on hand whenever I go into my reg. And yes....I have used my back ups many times.