The Zone System is most effective with sheet film, but that doesn't mean you can't apply it, at least partially, to roll film. I usually try to keep track of my exposures, and then develop for either the majority, or the most important exposures on each roll. Sometimes, I'll load two roll film backs for my RB67 with the same film, and use one for high contrast scenes, and the other for low contrast.
Edit: As usual, I've been beaten to the point.
^This - you can definitely use the zone system with roll film. Especially for something like 120 film as you can easily use a whole roll of film in a given shoot. Include some bracketing and that roll goes even faster. As a result, often you can apply development control to the entire roll. In cases where you will have more mixed lighting on a roll, I believe Ansel Adams "The Negative" recommends increased exposure coupled with n-1 development to ensure you capture the whole dynamic range, then printing on harder paper for negatives that need more contrast.
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