Preventative management is ideal. In a sense, reactive management leads to more of the documentation and other factors that you mentioned can lead to delays in progress. Preventative management is the best way to keep to a time and cost budget. However, it is possible to go through an entire project without needing any reactive management. Unforseen issues will take place in the project regardless of how much preventative action is implemented. Preventative management simply minimizes the amount of reactive management needed.