There was some research that suggested that sodium thiosulphate, ('acid fix') is inadequate for current emulsions and that ammonium thiosulphate is best. Further, alkaline solutions wash out much more efficiently than acid. I've been using a water stop for film for many years, (6 full changes of water) and an alkaline fixer. I ran tests a couple of years ago and it looked like the film washed to 'archival' in 2.5 minutes - I use 20 min though! The continued developer action in the water rinse is taken care of in your development tests, as long as you remain absolutely consistent in your technique. For printing, I use an acid stop but add a water rinse between stop and fix.