cutoff point (F3)

The f3 is the 3dB down point where the output of the speaker drops to 3dB below the average output. For high quality speaker systems for home or commercial use, it's useful. For car audio, it's essentially useless. If you build an enclosure that has a 3dB down point at 40Hz and install it in the vehicle, the 3dB down point will be much lower than the f3 with enclosure outside of the vehicle. Unless you a building a sound quality vehicle, build one of the enclosures suggested in the owner's manual for the woofer.
 
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.