I think it depends on a few factors, namely:

- The likelihood of the company selling (consider both near or long term) or going public

- The difference between 10% profit and the salary you would make with 10% ownership

- How likely profit is to increase compared to the likelihood of the company selling

In other words, it totally depends on the company and the situation. A web design company will probably never get acquired or go public, and so ownership doesn't mean much, whereas if you're working on something that interests Google or Facebook, the chances of acquisition (and the value of ownership) increase exponentially. Then again, my reasoning is based purely on monetary gain - you should probably consider the impact of ownership on the company and your personal life/career as well.