Typically, investors don't like it, and it tends not to be paid out in cash. However, I believe setting your books up that way gives you a better negotiating position when it comes to how low your salary will be, and possibly in relation to equity issues. It's a matter of having that sacrifice (no salary for X months) on paper so it can be respected as a real contribution with a concrete value, rather than just a giveaway that is somehow expected of a founder.