mike, i have a Bf109 coming up and from what i can gather, the upper splinter cammo is sometimes a hard edge, but the fuselage sides are very soft. if you can get a nice, tight, fine line and your aim is good with an airbrush, you can get close in and nearly mimic a hard edge. going slow enough to cover on the first pass and holding the brush steady is the most difficult part of achieving such lines though.
RAF birds were definetly hard edged. from what i gather, they used rubber mats to mask with in real life, and had templates for each type of bird. thats why all the cammo schemes on a spitfire are nearly identical. and then again, its your preference too. i like the soft edge myself, but if i'm going to model a certain subject and want to capture the realism, i'll do whatever to make it look as close as the real thing as possible. i'm just beginning to really build seriously, so i'm still in the experimental stage. later.