I did this when I suspected my ex. In my case, although the tracking threw up one suspicious tracing, and another where the phone appeared to be switched off when he went to a particular area, the tracking didn't actually help me. It was later, different evidence which emerged which confirmed my suspicions.
Some people will say that it is never ok to track someone, but what if you have really strong suspicions and you have asked your partner but they are not forthcoming about admission?