I have 9lb schneider as my main line with 15lb or 20lb schneider (whatever I have laying around) as my casting leader.
Is all about maximising distance without compromising ability to land fish. In mono lines, 9lb with a 15 - 20lb casting leader is the lowest poundage I deem that can be used safely on the likes of big tailor (10lb +), longtails, spaniards etc in the surf, but still achieve good distance casting without throwing lures off. I've used 6lb schneider several times and have beached longtails 8 - 10kg, broadbarred and tailor around 8lb, but have been busted up a fair bit in the process - which doesn't happen with 9lb. The leader knot is more vulnerable to break with 6lb.
Lindsay