Your company will have set the strike price, so check with them on that. I think companies all behave differently when it comes to vested, unexercised options at the time of an acquisition or IPO, they may give you an opportunity to exercise them, pay out a (likely) small amount for them, or they may just disappear. My basic thinking is if the strike price is really low, as it probably is in a startup, you may as well go ahead and exercise them as soon as you can, especially if you don't have that many options and the total cost is minimal. You should also ask about the fair market value of the shares (though they may be the same as the strike price at this point) because if you exercise a lot of options and the difference between the strike price and the fair market value is significant, it could have alternative minimum tax implications.
Just be aware (and it sounds like you already are) that the chances of the stock ever being worth anything are pretty low, so figure out how much money you're willing to gamble on the future of the company.