WWYD: A Job on the Side

Am I obligated to let me primary employer know if I take on freelancing work (be it career-related or just bartending or something) on the side?

The Hustle of a Doll Maker: A Chat with Cinnamon Willis

Cinnamon, a former coworker of mine, started making dolls over two years ago. She works out of her Bronx one-bedroom at night, after coming home from her full-time graphic design job.