If the company is public, then you can essentially leave whenever you want, exercise the options and sell the stock to pay the costs (exercise price + taxes).
But if the company is private, you have to pay the exercise price + applicable taxes (which can exist even if you only have theoretical gains) yourself, without the ability to hedge your risk and sell the still illiquid stock. If you have ISO stock options, you have 90 days after you leave (or are fired) to figure this out or lose the stock options altogether.
So if you are joining a company with the following combination of elements:
1) High exercise price (the math is: # of options * exercise price... is this a lot of money or not)
2) ISO stock options or the stock option plan gives you limited time to exercise after you leave
3) No reliable system to sell the private stock
Then you should add in a further discount on the stock options, because there may be situations where you cannot afford to reap the benefits of the options if you leave (or are fired) before there is reliable liquidity for the stock.