Technically UK lighting regs say it should be centreline or DS, and it's probably marginally more visible, but in practice it's unlikely to make a huge difference.
I ended up going centreline anyway as I discovered the mount I'd made to put the light at the brake bridge on my last frame would work bolted to the mudguard.
For dropout jointing a rear light, driveside dropout or does it not really matter? There’s more options on the non driveside for me to choose from.