I know that there will not be a simple yes/no answer for this, but I recently booked a BA Holidays booking for flight+car. When making the booking I used the "Create custom trip" option and went through and selected the flights etc and set the pick up and drop off time for the hire car so that it was exactly 5 days.

I am now looking at the reservation in my Avis account and it shows pick up at 13:05 (the same time that my flight lands) and drop off at 15:15 (two hours before my flight takes off) and also shows that it is a 6 day booking.

Given that we will have checked bags to collect and there may be a queue at immigration made longer by the fact that I am travelling with non-EU passport holders I would have thought that delaying the pickup to maybe 14:00 would have been fine (yes maybe have too hang around in the airport for a little bit) and I would also have been happy to drop the car off on return at maybe 14:00 thus making it exactly 5 days hire instead of 6 and maybe reducing the price but despite me selecting 14:00 as both the pick up and drop off time, it has given me the other timings and made it a 6 day booking, I could quite well be paying a whole day extra car hire for a matter of 1 or so hours.

The booking is paid in full so I guess there is nothing that I can do about it now, but maybe something to look out for next time.