You should usually be doing 'data handling' wherever results in the best user experience. Persistence and calculations which are essential to the integrity of y

You should usually be doing 'data handling' wherever results in the best user experience. Persistence and calculations which are essential to the integrity of your application's data obviously need to be done (or at least validated) on the server, but if the application has any kind of complex workflow then constantly going back to the server to generate new application states is likely to severely bog down that workflow.