"Sour" to me only occurs when a pipe is smoked wet, left wet, and smoked again cyclically with no drying out or cleaning in between. There's a huge difference between a "dirty" pipe in need of cleaning and a "sour" pipe in need of cleaning and drying. Pipes are dirty after use, that's just the way they are. But if you smoke well and have a dry bowl at the end... you won't ever have a sour pipe.
It's not to say that smoking a little wet would ever wreck a pipe - briar is tough stuff, but it needs to air a bit between smokes or eventually it will just bog down.