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That depends on the head unit. If your head unit connects up via the dock connector then the volume on the ipod doesn't matter. If it connects up via the headphone jack then it does matter and I would tend to think full volume would work best.
I use a Harmon Kardon Drive+Play in my '06 and it has a dock connector, separate screen, and a separate controller. It connects to my stock radio. Sounds clunky but it really isn't and I like it a lot. My wife has a Sony head unit with dock connector in her '98 and all the ipod controls are built into the radio.
75%, then adjust via the head unit. that will provide the least distortion and is pretty much the rule of thumb when hooking your ipod up to any outside speaker source