I think they should have probable cause. It is not only a waste of time if it ends up being nothing but it also takes the police away from matters that might be more pressing and more important than just fiding location data on someones cell phone. They should have a good reason.
Yes, probable cause should be required for police to use cell phone location data, because collecting cell phone data is a form of a search. Any time the government collects information about citizens it should have to have a good reason. There should not be any general data mining without probable cause.