Yes, the heating/ quenching will resoften the copper washers, (as well as aluminum ones too), but don't expect it to return them entirely to their origional dimentions. Say they're squashed .010" at the circular "mark" where the banjo hose fitting squeezed them. That won't return to the other unsquashed areas dimentions. I've had no issues using resoftened crush washers, but I've also not had issues just reusing used ones. Just look at them ... don't want any severe deep marks in them. Look at your new hose banjo ends. Sealing edges ok? No factory defect?
I've found some washers are sized poorly... their ID too big so they fit sloppy loose on the banjo bolts OD. If so, try to "center" them and the hose-end on the bolt for complete surface contact area.
Oh BTW, just to be sure the bolts not too long, (stuff happens),.... hand assemble them with two washer left out. The bolt should still pull up to the hoses end. (DON"T TIGHTEN WITH A WRENCH... your just checking for proper fit). Also, doubling up on the washers where one normally would be, should never be necessary... one washers plenty thick enough. And orienting the "stamped" curved edge towards fluid flow direction ... or any other orientation for that natter,is also irrelevant. Complete/full mating surface is what you want to assure.
Last, if problem persists, might want to measure the end of the hoses width... around it's circular perimeter, (micrometer or calipers needed). Maybe there's too much width variance for the washer to accomodate.
Dave