no, the cable supplied through nissan/infiniti that connects to the ipod dock will give you the best signal.
you may want to try a new cable, i don't believe they are too expensive through ebay or other sites, but the line level output from the ipod dock connector will give you a better audio signal than the headphone jack. The headphone jack through RCA cables may sound louder (if you crank the ipod's volume out), but technically, the audio quality will be better through the dock and cable.
"The difference between the two here is that the line-levelís signal is noise-free and clean because it doesnít pass through the iPodís crappy amplifier. That crappy amplifier adds unsavory extras (especially at higher volumes) instead of allowing the music to come across flat and true to the recording. Thatís why itís best for music not to have to go through it."