The research that I have been exposed to indicates that the reason for acid (either in the stop bath or in the fixer) is for the purpose of halting developer action. The control of the length of time in developing is important to me but I do realize that others compensate in other ways. The addition of Sodium Sulfite to the Sodium Thiosulfate in the fixing bath is that the Thiosulfate in reaction with the silver halides produces free sulphur as one of the by-products. The sodium sufite combines with the free sulfphur to produce sodium thiosulfate. As I understand it this is a secondary part of the process and effectively lengthens fixer life. I don't have any experience with the alkaline fixers, I do understand that some pyro users use an alkaline fixer to limit stain reduction. I have, however, not found stain reduction to be problematic when using ABC Pyro in conjunction with Acetic acid stop and hypo.