I usually go 1/4 turn, never half a turn. 1/8 turn is probably fine too. Some people even go as far as to make it .002 lash or something stupid that just creates slop and noise and extra wear in the valvetrain that defeats the purpose of using a hydraulic lifter. On the other end of the spectrum people have gone 3/4 turn, but I don't see the point in that.
Like Martin says, its a metal locknut, as long as the locknut isn't wore out it doesnt matter where you put it, it should stay there. You are just changing how much you proeload the hydraulic lifter.