My 629 5" Classic is a -6 also. First model with the lock & my first S&W. The only time I monkeyed with the lock was when it was first bought, I checked it to see how it worked, and not used since. I've put approx. 600 rds. (44Mag./44Spl.) thru it and have never had a problem with the lock.
I put a Wolf spring kit in it and it has an absolutely awesome double & single action trigger.