What's the worst that can happen if you have to change or cancel your order? Do you lose a deposit? If you have to change some cabinets, can you get all the others at the sale price?

If there's no downside to this, go ahead and do it.

That promo has been running in one form or another since December of last year. When it expires, it will be renewed again. You are not under any great pressure to decide now, and there is NO way I'd ever recommend finalizing any cabinet order until the drywall is up and you can take final measurements. There are way too many possible changes that occur between now and then, and if you sign off on that order based on plans, and the dimensions aren't correct, then you are the one that ends up eating any errors here, plus the extra time to reorder for the mistakes. If the installer comes and does the final measure, then any errors are on his dime. (Don't forget that in most states, if the company that sells you cabinets also installs them, you do not pay sales tax on them.)