Depends on which a/c, era, JG etc.
Most of the a/c pics I have seen show the demarcation line starting to 'creep' up onto the fuselage, and a semi soft/hard edge.
Found a few pics showing a squeaky clean edge, and some showing a mongrel overspray edge.
Depending on the scale, lke 1/48 that I do, I make my edges a little soft but mostly hard, using some Mask It, and spraying at an angle to suit.
On larger scales, I'd go a hard edge, lapping up onto the wing root/fuselage, then apply a little blending.