You don't have to reset, but some people find that a factory reset after a system update makes things run more smoothly. If you don't want to go through the whole process of factory reset and restoring the phone afterwards, you could try wiping the cache partition instead. The steps for the Moto X should work for the Moto G: https://motorola-global-portal.custhelp.com/app/answers/prod_answer_detail/a_id/94946/p/30,6720,8696