washing it after blasting is additional insurance, if your worried about flash rust that can occur just from humidity, if it does hit it with 80 grit on a DA and get rid of it.
After blasting there can be contaminant on/in the metal-it needs to be scrubbed off, dawn works well because it doesn't have abunch of other things in it, scents, etc.....& it leaves no residue if rinced properly.

I've painted many parts with epoxy after just a dawn scrub, no epoxy failures. Wax & grease removers won't get everything , the cleaner the part you start with the less W & G work necessary.

If you do a lot of DA sanding for paint removal that can force residues into the metal - don't worry about the water rusting a hole in the car just make sure it clean prior to any paint. All in the prep....