Would you recommend doing unpaid jobs if (an only if) that's the ONLY way of getting a proper portfolio?
Nope, I wouldn't. I'd recommend REALLY cheap jobs instead because at least then there's still an exchange happening. Working for free just hurts the overall impression of audio people's worth.
Edit: Start cheap to build up some credentials and experience. Then raise your rates. Don't fall for the "this will be huge exposure" line that some devs throw out there because, too often, the game isn't nearly as successful as they think/hope. And that's if the game is even finished!