i say decal now, wash later. don't forget to cover the decals
with some more clear gloss after you're done putting them on. i
know of people weathering first, then applying decals and adding
scratches+drybrush to those as well but i think the markings are part
of the base paint job + camo + whatever so they should be treated as
such. plus the decals might not go on so well over the oil.
is this the finemolds x-wing? if so, good luck with all the
decals. there's about 50,000,000, with half being small dashes
mm's apart. when i did mine, i actually made most of those marks
with a drafting pen. check the sci-fi section for "my old x-wing"
topic if you'd like to see how it came out.