Car/iPhone Audio Setup

In my car, the setup for audio necessitates the use of the 3.5mm headphone-jack instead of the 30 pin dock connector. Since I can change the volume level on both the device and the car stereo, which is the ideal setting? I notice that if I change them independently, the audio quality changes. More volume on the iPhone and less on the stereo produces more bass but it seems more washed out. More volume on the stereo and less on the iPhone produces no bass at all and distorts the audio and is generally too quiet. The middle ground is average but has the less significant problems of both. Does anyone else in a similar situation know the ideal levels of each. or is it merely preference? (If it helps I'm using iOS 6 on an iPhone 3GS)