Is there any guarantee of a cash-out opportunity at private startups? Or is it at the mercy of your employer and one can even have bad experiences like Zynga where your options could be clawed back?
That being said, read the options agreement carefully and ensure your interests are well protected.
It's worth noting that, in nearly all cases, you will not receive an options agreement until well after you have accepted the offer.
I've never tried to ask for an options agreement prior to accepting an offer, but I'd imagine it would be a very difficult thing to get at most companies.
Point is, negotiate a salary that meets your needs, and treat the stock options like lottery tickets, because that's what they are.