I always thought that a stop bath is required when using neutral fixers, but several posters here claim the opposite. The label of the Rollei RXN Neutral Fixing Bath states the following:
"A stop bath must be used before fixing in order to stop development and maintain the neutral pH-value of the the fixing bath."
It seems logical to me that an acidic stop is necessary to halt alkali developer, so when using a neutral fixer an acidic stop would be required. And if no acidic stop is used, then alkali developer will be carried over to the neutral fixer and make it alkali too. Of course carrying over acidic stop bath would lead to the fixer becoming more and more acidic with time. Rollei actually additionally recommends rinsing the print briefly in water between stop bath and fix to avoid this problem.