There are no hard/fast rules but generally you'd go heavier for longer distance work and lighter for close-in stuff.
I personally am not a fan of back leads. They reduce sensitivity and if there's any weed or debris around they can cause big problems. I remember watching a lad having to land a fish in a boat a few years ago.... his back lead had snagged in the weed. Once released, the rig/fish was also weeded up. It was like a spiders web of line around his swim, looked like a nightmare. He lost the fish.
If I had to use one, I'd use a flying back lead. The important thing is getting that last few feet pinned down.