Porting doesn't affect accuracy at all. Porting is meant to dissipate gas to lessen the recoil on a firearm from a large burst of gas as the projectile exits the gun(it'll force the gun upwards otherwise). Since the pressure behind a paintball is no where near large enough to give any considerable recoil that would require porting, it doesn't affect accuracy.
On the other hand, porting reduces sound. If you like a quiet marker, then get the porting.
