Just to round out the earlier mixing discussion for those interested, I went through Haist's chapter on mixing photographic solutions again. In the context of mixing order, while he does specifically mention that Metol will precipitate out if mixed into a Sulfite solution, he does not talk about adding a small amount of Sulfite to scavenge Oxygen before the Metol goes in. Both D-76 and D-72 are used as examples in the text (both containing Metol and Sulfite) and in neither case is any Sulfite added before the Metol.

Of course, Haist is not the only credible source so who knows how the "pinch" practice came about. I'd have to defer to the chemists.