Chitting earlies is definitely recommended, but it's not so crucial for maincrops. If you forget to buy your seed potatoes until the last minute, just bung 'em in anyway. They'll grow! They'll probably have chitted in the shop anyway, though it isn't quite as good as when they're nicely spread out. If you get them with really long, skinny sprouts, try to plant them without breaking them off. They'll still come good!