Whiteys are in healthy numbers here on the Truckee. And I agree with your sentiment about them being helpful in selecting flies.
Here in western NV, they tend to school up for most of the year, so it's common to catch several in one slot, so the challenge becomes getting past the school to find trout nearby without spooking them. That said, last spring I pulled a 20" whitefish (the biggest i have caught) out of a slot, and then had a nice brown on the next cast in the same run.
The bigger ones pull like tanks. Fun if you ask me.