I use the Visa code only when using a free weekend day coupon also sponsored by Visa. The reason is that in order to get the primary rental coverage using the Saphire Reserve, you have to pay the rental entirely with your card. It has been mentioned in the past in other threads, that certain coupons, including Emerald Club free days could invalidate that coverage, because Visa's insurance company interprets the "free" day as a different payment method. By using a free day coupon sponsored by Visa, it would be much tougher for them to say that I didn't pay for the entire rental using my card.