I trim leaders religiously for my IIIC now, but for years, didn't know to do this, and never had a problem. Understand I was running a risk, though. Having exactly the right amount of slack leader out of the cassette is the trick, I've found. You want to be able to form a curve in the film leader that mimics the curve of the film path in the camera before you slip the film in. You learn this by experience. If it doesn't feel right, don't force it, be patient & pull the film out and try again. Make absolutely sure the film is tracking "clean", no binding, sprocket holes aligned with sprocket teeth, before closing up the bottom. Waste a few extra frames of film if necessary, it's worth it.
Concerning repairs: this fellow's not in New England, but he's absolutely one of the best for screw-mount Leica in the USA. Doesn't do M mount or Leica reflex, just screw mount:
L & J Leica Repair
109 Royal Oak Road
Greenville, SC 29607
Best time to call him is late afternoon weekdays, in my experience. He can take his time, but his prices are OK, and the quality of his work is totally worth the time it takes. He's also a complete gentleman.