Costco typically sets up a dedicated register for check cashing only when the rebate checks first come out. They are perfectly happy to give you cold, hard CA$H. Do this. Do not use it to pay for your purchase. Using the rebate check to pay for your purchase means you won't get the rebate on that purchase. You would be effectively overpaying by at least 2%, as MM_MG pointed out. The ten dollars you save, with virtually no effort, is the equivalent of roughly 6.5 Hot Dog & Soda lunches at the Food Court. (Or whatever you want to do with ten bucks.)
BTW, you can cash your rebate in the checkout line if your cashier has enough money in the till. Put your purchases on your Costco Visa, then cash the check afterwards, while you're still at the checkstand. Just be sure you state your intentions clearly so there's no confusion. The cashiers are all experienced. They're used to this cycle and they do not need to ask permission, provided they have enough money to cash you out without depleting their drawer.
Source: my brother and my BFF's DH are both 25+ year Costco employees. That's what the silver name badge means. If you see a person wearing one at your Costco, congratulate them on their service. It always makes them smile. While they may not be super early retirees, there's a very good chance that they're millionaires.
