In my humble opinion, the F3 of a box in car audio can be helpful if ou're trying to match the roll-off of the box to more smoothly meet the roll-off at the bottom end of your midrange or mid-bass drivers for a more seemless transition. Granted, the crossover points will affect this as much as other factors like cabin response and transfer function, but transfer function for a specific cabin can be determined, crossover points and Q are chosen, and knowing these things along with the anechoic response of the drivers you choose will help in enclosure design if you worry about smooth transitions. (SQ, as Mr. Babin noted) If all you want is loud bass then sure, it won't make much difference since when you design a 4th order or 6th order box for peak output, the F3 doesn't much matter as you'll be more concerned with the modeled response and less concerned with getting a flat response or a musically ideal tuned frequency with the ported side output.
If you do want a SQ system, the F3 will matter because you want to know where the speaker will drop off naturally (in it's enclosure, and in the vehicle) so as to pick the proper Q and crossover point @ the crossover.