i guess to use the car with woofer systems and 5.1 systems. one needs to get the car sound proof so it does not rattle in any way at high bass and treble volumes. local manufacture cars rattle even if you whistle in the car.
so yeh i guess the sound proofing should be done. but only if its rattling if not then no worries at high volumes.
just a piece of advice, keep the volume of your car audio to what suits you and people around you, being a nuisance in traffic and playing it too loud is a hazard.
good day