In playing the math game in audio, I would recommend wiring them in series. This way you can take them down to 3 ohms and still maintain a usable impedance on the amp. Plus you'll get the added benifit of actual usable output.

Taking most amps into a higher impedance (in parrallel it would be 12 ohms) you might over stress the amp (trying to get reasonable volume from your setup) and then trip the MOSFETs (or whatever their using today). Not that this will ruin the amp, but repeated tripping of the current reading ICs will eventually render the amp useless.

All being, most car audio is designed to be stable at 2 ohms anyway. My biggest peave with that is that sonically, 2 ohms slaps the speakers instead of driving them, not to mention limiting the actual headroom.

Hope that helps...