Here's my two cents. When you decide to record your music, the two most important things in your studio must be your tranducers; your microphones and your monitors. Your microphones have to be the "very best" you can get your hands on (which might be subjective) because you want to reproduce whatever it is that your picking up as "faithfully" and "realistically" as possible without enhancing anything. Your monitors, on the other hand, should be as good as you can so that you can be able to hear the "truth"; you want to hear the exact frequencies, as clearly as possible so you know exactly what was recorded and how to blend (mix) all those priceless takes together to make your final product as enjoyable and exciting as you can. Now, listening to your mix in the car is good. I remember hearing from a few very good engineers that the car is a very good place to check on mixes... but remember that not all car systems sound alike, and that the speakers in your car are not meant to be flat, so , just as an example, an ultra deep bass that sounds wicked in your JBL's, Alesis, Roland's or Tannoy's might distort or sound strange in your car's kenwood's or Pioneers. So you might have to adjust to be able to be happy with that deep bass sound in both set-ups, or even try a small boom box as well. Keep in mind always that 99% of the people that will get a hold of your mix will play it in systems that do not sound flat and 4 out of 5 will play it in a lousy soundind cheap-o stereo. So what sounds awesome in your studio monitors might not come through so well in cheap-o stereos. What I do, is I have two sets of monitors in the studio. I have Roland DS-90's as my "main" set and when I'm happy with a mix and think there is little or nothing else to do with it, I switch the audio to a pair of Roland MA-8 stereo micro monitors which I believe are much closer sounding to what most of the people that will get a copy of my mix will use to listen to it. Generally I'll have to tweak a couple of frequencies here and there (by cutting, not boosting) after the listen of the micro monitors. Once I'm happy with the sound in both sets of monitors, he next step I take is to burn a CD with it and listen to it in three systems, first, generally I play it in a small boom box I have in the bathroom while I shower; second, I play it in my home stereo in my living room; finally in my car. Always making mental notes of what sounds good and what doesn't in the three diferent environments, and then I go and adjust a little bit more in the mix to get my FINAL MIX.
Best of luck,
Carlos