jdef, yes, I beleive there is still risk of over fixing. 4 to 5 minutes is not excessive even in an acidic fixer so you are safe.
David, Ah I see.....you know if the stain is proportional then I would be afraid of dissolving silver in the shadow zones. I am still not clear why an alkaline fixer would not dissolve image silver. I wish someone could give us the formula and an explanation. As a matter of fact, if you have a bottle David why dont you check out the ingredients, I beleive they have to list them even if they dont put the ratios.