It won't make a huge difference either way.
As long as you have enough pump, each injector just sees the rail above it as a "plenum." There will be a tiny pressure drop every time an injector opens, but that's offset by a tiny pressure surge every time it closes. You'll end up with a pretty constant average pressure that's determined by the regulator. If your supply line is the same size as the ID of the fuel rails, it also won't make a difference. If you feed with a larger line, then parallel makes more sense.
If you don't have enough pump, and pressure starts to drop, it becomes a bigger issue, and you're going to impact the furthest out injectors the most.
All that said, I run an AN8 feed to AN8 rails in parallel, and then twin AN6 returns to the regulator with a single AN6 back to the tank.