You can specify your scheduled flight arrival time, or you can add in some time to retrieve your bags. It really doesn't matter. People do both.
Your actual arrival at the rental car area is unpredictable. The rental car companies know this. Flights routinely arrive between 20 minutes ahead of schedule and 30 minutes late (and sometimes much later). The wait at baggage claim can be anywhere from almost no wait to an hour, depending on how backed up the baggage handlers are and weather conditions (such as thunderstorms).
The time you specify only becomes an issue if it might affect your return time AND your rental car company pre-prints your paperwork and leaves it in your car (such as Hertz Gold). Rentals are billed in 24-hour days, with (usually) a 29-minute grace period (may differ by company). So if your paperwork shows a 6:10 p.m. rental, but you return your car at 7:00 p.m. a few days later, you would go over and pay for an additional hour (typically half the daily rate). But if your paperwork shows a 6:45 p.m. rental, you might still be within your grace period.
Make sure you know the policies, rates, and grace period of your rental company rather than relying on my description of "typical" rentals.
PP has some good points. Play around with your pickup/return times since you may be able to save some money by keeping to the 24 slots. I matched up my pickup time and will be saving a lot even though we'll wait in the terminal a little (planning to eat lunch there now to kill the time.) We're not heading straight to WDW, so we're not in a rush.
Your rental is 9 days. You would pay for (1 week @ the weekly rate) + (2 days @ the daily rate) + (all sorts of fees and taxes).
To answer your original question, I would just use the scheduled flight arrival time. That's what corporate travel departments typically do. Or if you're booking on a web interface with 15-minute increments, just round forward to the next increment after your scheduled arrival.
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