interesting. I'm basically with BCRider and zzyzx_xyzzy here for the reasons they described... in a lot of cases you won't have actual symmetry.
but on the other hand, Sheldon's method *does* seem a bit dodgy and also leaves room for its own errors.
I'd probably use either method on cheaper bikes and feel fine about it.
I've used Sheldon's method on a reasonably-light 1980's steel road frame and things worked out fine.