It is best to be clean prior to practicing. The yoga sūtras mention cleanliness (śauca) as a main pillar to the practice of Yoga. It is understood that many people will be arriving from different places (work, family, exercise), that being said please be mindful of your cleanliness as that will affect your own experience. It is also beneficial to cleanse after practice. By bathing you will cleanse the toxins of your body and be able to focus better on your tasks.