That said, it's irrelevant if you exercise and sell an ISO at the same time, because short-term cap gains are taxed as income tax, too.
The real interesting term is having the ability to early exercise your entire stock grant, before it vests and before there is a spread between its strike price and fair market value. (The company would have a repurchase right, to implement the vesting schedule.) That way the cost is very low, and there's no worry of having to exercise in the future when it could be taxed unfavorably, and the clock for long-term capital gains starts early.