The closer the better. First choice is right next to the mids. Second choice is in kick panels down low near the mids. Third choice is higher in the door, and last choice is in the dash. According to an install document I saw while checking out some comps (Infinity Perfects, I think).
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's