Any method that works only with the model data and is used in multiple places in the application should be a part of the model.
Next, if you have functions that operate on multiple pieces of data (or more than 1 model instances) you should put them in the view where they are used. But if these functions are needed from multiple views, then personally I create a helper class to hold them, and the helper class is outside the view and model files.
Of course in the end, none of these are hard and fast rules. Just go with whatever you feel is more maintainable, and use these as general guidelines.