Likewise, he added, "If you're someone who's working at a nuclear power plant during the day, I think your employer's entitled to say that's your only job. You shouldn't be out at night leading fishing tours."
Some employers maintain a no-moonlighting policy. Others have a non-compete policy that forbids employees from working for industry competitors. Understandably, they want your undivided loyalty and they want you to keep their trade secrets quiet.
Even so, some workers find their employer's attitude toward moonlighting oppressive.
Take Michael (not his real name), a freelance designer in Cincinnati. Before striking out on his own in December, he worked for a company that allowed its creative staff to take freelance jobs paying less than $10,000 -- pending their manager's approval.
"The catch was that the manager would never take time to officially approve your request," said Michael, who moonlighted first for the extra cash, then to build up his own business. "So you either lost the client because of the long approval period, had to let the client go because the wait was embarrassingly long or just said 'screw it' and did it."
While it's rare to see a company sue an employee who's been two-timing them, if you don't abide by an employer's moonlighting policies you do run the risk of being fired, Robinson said.
A New York marketing professional I'll call Sarah is willing to take that chance. Although she isn't sure her employer would approve of her writing research reports on the side to line her savings account, she likes "the security of knowing you haven't put all your eggs in one basket."
Given the economy, she said, "It really makes me feel a lot less nervous about getting laid off from my day job."
This work is the opinion of the columnist and in no way reflects the opinion of ABC News.
Michelle Goodman is a freelance journalist, author and former cubicle dweller. Her books — "My So-Called Freelance Life: How to Survive and Thrive as a Creative Professional for Hire" and "The Anti 9-to-5 Guide: Practical Career Advice for Women Who Think Outside the Cube" -- offer an irreverent take on the traditional career guide. More tips on career change, flex work and the freelance life can be found on her blog, Anti9to5Guide.com.