Hmmm. I am not talking about the effects of aging... I am talking about the uneven paint coverage, freehand "straight" lines, etc that are very different than the typical decal produced with a vector art program with mathematically perfect circles, arcs, lines, contour widths, etc. I'll give you it probably isn't a concern for 1/72, but in 1/32 where a couple of us work, it is maybe something worth noting.
I spend a lot of time looking at photographs when designing decals and yes, sometimes vector graphics straight lines are too straight and circles are too perfect but if we are talking about German crosses and allied cockades, the vast majority
were pretty near perfect - especially when reduced to 32nd scale and lower. Modified German crosses done in the field during the confusion of transition from eisernes kreutz to balken kreutz were often less than perfect, to be sure and cockades painted in the field could be rough looking, but that was usually due to poor paint coverage, not the evenness of the circles - they're easy to achieve with a nail, a piece of string and some chalk!
However let's not forget that, in the years of the Great War, sign writing and "commercial" art (such as seen on the sides of French houses, for instance) was far more widespread than now. To become a professional sign writer took many years of apprenticeship, so clean straight lines and smooth curves became second nature and these professionals were valued in the armed forces (see Bruno's very interesting post above) where decals and stencils were very much rarer than in later times. For such things as serials, house "styles" evolved for the likes of Sopwith or Albatros, for example, but they were hand painted and although they may have shown differences from aircraft to aircraft, they were, none the less, well formed figures. Some other manufacturers such as Pfalz, for example, used stencils extensively and a number of SPAD subcontractors and some other French manufacturers (Breguet, for example) used decals.
As to uneven paint coverage, this is almost impossible to recreate using conventional screen printing but for the skilled modeller, over painting decals with different effects can achieve the look with a little practice.
The above sounds like I'm being defensive and yes, you did strike a nerve but I'd hate people to think that I (and I expect my fellow decal designers - particularly Malcolm Laird at Wingnut Wings ), aren't alive to these issues.