There are two occasions where I would recommend a factory reset. (1) After a major OS upgrade (Note: This may not be necessary, but is good to do for optimal performance and user experience). (2) An issue or issues have risen where nothing else has worked to rectify the issue(s).
I do recommend wiping the cache partition once or twice a month to help keep the system running at it's bestツ
On a side note, I personally do a factory reset after I get a new phone, but I usually play with it a little while to see all what came installed with it and how everything was setup from the factory before doing so.