I generally take the approach that each run mode should do one thing. How you determine the granularity of the 'one thing' is up to you, but by and large I tend to define my 'one thing' is 'what do i need to do to get the user to the next screen?'. So, most of any given run mode I code is geared towards retrieving/manipulating/authenticating data to feed to the template i want to serve to the user. And unless the user is doing the same exact action again (not the next 'bit' of the same action), they won't be revisiting that run mode.
That said, I do at times go one step further, particularly for important forms. I'll dedicate a run mode just for the processing of the submitted data, and then send the user an HTTP redirect to the next mode. This is so that the user doesn't accidentally resubmit the same data with a refresh when they're on the next page.
