Never heard that. I do know that RC paper can stain at the edges from being in the toner too long.

The point of toning is to convert the metallic silver to silver selenide (or some other compound depending on the toner). So you want ALL excess silver GONE. That means you fix, hypo, and wash. You want all that undeveloped silver gone since it can stain when toned.

Or so I understand it. I could be wrong.