The approach depends on how the aircraft was finished - an early war aircraft with clear doped linen can be quite transparent, so you would often use preshading to create the effect of ribs and structure showing through the fabric. Later war aircraft were often painted in protective coverings which were designed to reduce sun damage, so these would be opaque, thus preshading is not really appropriate (but people who like an "artistic" finish do employ this). WWI aircraft were generally not dirty, they were kept clean because external dirt impeded their performance. There would be signs of "grime" and general wear and tear however.
Some aircraft show a lot of evidence of oil staining, especially rotary-engined aircraft, but most of this comes from oil working its way along the internal structures and soaking through the fabric from the inside. This seems to be commonplace on the fuselage of the Nieuport IV for example. There's an article on how I did this on a Fokker Eindecker in the Great War SIG newsletter here:
https://drive.google.com/file/d/1JuZay4W4YAISJ7EcMsQGHHnrSoTxJQWu/viewBottom line - find pictures of the type of aircraft you are modelling to identify what techniques are appropriate and let that guide you.
