I guess it depends on the rental agency. The other day, at Thrifty HNL (probably the crappiest rental agency in the universe, but usually the cheapest there), the lady wanted to charge me for putting my QF points number on the contract, even though no car rental agency ever charges for foreign airlines points/miles (because they're not liable for the 7.5% mileage tax with non-US airlines). I usually don't bother giving them my FF number upfront but call in the info later (I'd have to do that anyway in 90% of the rentals).