There was an error when my response was inserted, and not all of it got inserted correctly. Therefore, I am trying this post again.(posted 8367 days ago)Yes, you can just rinse the film or paper thoroughly in water. However, I would recommend using stop bath. Stop bath performs several functions and should be used with all types of film and paper. Developer that is carried over into fixer can greatly effect the final film or print image.
Developer is alkaline (pH over 7), while fixer is acidic (pH under 7). When the developer is carried over into the fixer it exhausts the fixer rapidly. However, the cost of replacing the fixer is not the issue. As the fixer becomes loaded with silver from use, the change in pH caused by the developer carried into it can create dichroic fog on the film or paper. Worse, than that, some of the developer carried in the emulsion is oxidized, and this can react with the silver in the fixer and staining can occur. If the paper or film becomes stained, you will not be able to wash the stain out as it is composed of insoluable by-products. The stop bath neutralizes the developer and oxidized developer in the emulsion and prevents dichroic fog and staining.
Acetic acid is used because it has a unique property. Acetic acid stays at a very stable pH over a long period of time as it is used up. There is a limit to the amount of ions that will go into solution at any one time in acetic acid. As the ions are used up, more will become available keeping the solution working at a stable pH. In effect, it is "self buffering."