But...he'll only grab ONE cheap craft on his way to the workshop, won't he?
Yes. And if he then proceeds to encrust a lot of crafts without interruption, he'll be encrusting the good ones. You just need to make sure he doesn't decide to grab a cheap craft, encrust it, then decide to go get a drink, then grab another cheap craft near the drink stockpile, encrust that, then decide to take a nap, then grab another cheap craft near his bedroom, encrust it, then decide to attend a party....
just to be clear (though I doubt the asker is even around anymore), this doesn't happen with gems. the gem setter has to grab the gem first, not the item to be encrusted, so he won't grab some random craft and cart it to the other side of the fortress. he'll go get a gem, take it to the workshop, and then take the nearest craft.
the only way to get the result you describe is if he already has a gem within the workshop that you had previously forbidden for some reason and then unforbid. if the gem is already there, he'll realize he just needs the craft and grab the nearest one. but the result you describe, of this happening over and over, is extremely unlikely.
this is true for all decorations I can think of. studding with metal, decorating with bones, everything that I've done, the dwarves collect the decoration first. unless that decoration is already within the workshop for some reason, that means they'll always decorate the items closest to the workshop.