Depends on what techniques you may be using. There are some, such as pre-shading, which you want to do before the decals go on. Matter of fact it's done before the paint's put on. (Pre-shading is highlighting the panel lines and any other desirable areas with black or some dark color which will stand out when the paintjob is applied)
As to washes and pastels and all, most folks who weather their models do all this at the very end after everything else is completed on the model.
I tend to mix it up a bit; in areas where there's no markings I'll do a little bit of something before I put the gloss on for the decals. Then I'll do some while the gloss is on, both before and after applying the decals and a secondary gloss coat to seal them. Then I'll do some more after that secondary coat of gloss is applied and, finally, I'll finish up after the final clear is applied.
What this does is to change the tone and/or value of the colors I apply as weathering. Each successive application of clear will alter the appearance of whatever colors are below it, including washes, pastels etc. This is just my own technique, based on my observations of the real deal, my interpretations of those observations and, most importantly, my personal preference.
My advice is to try a buncha different techniques and ways to apply those techniques and see what works for you. Ultimately it comes down to what
you like.
Fade to Black...