I did it purely for experimentation, not necessity, since these were indoor hydroponic plants. There were no bugs or dust to wash off. Gently touching a finger to the plant leaves a gooey sticky residue. Rain dose break some trichomes but it is part of the outdoor grow tax. Don't see how a cannon ball into a bucket wouldn't knock a few loose.
Washing won't ruin the bud. I would gladly give up a few cannabinoids if smoking buds with manure dust was the alternative. Nothing ruins a smoke sesh like discovering a bird left a present on a bud. My experience was washing a clean bud, did not improve it, and it had a minor negative effect.