Is there any reason to NOT take deliveries of cars in GT5?
I think the reason they've implemented the "Delivery" system is that they're keen for you to have at least one look at all the cars you receive. The whole GT Life mode seems geared towards giving the player an experience of a range of cars throughout history, so they don't want you to have cars just appear in your garage that you never notice.
So no, I don't think there is any reason to refuse delivery of a car.