Okay, think of it this way
How are you going to wash it for the rest of it's existance?
Because that will be the biggest determining factor.
Once it is washed at a certain temperature it will be set at that sizing if you consistently wash it at that temperature. There may be miniscule trace amounts of shrinkage, but that will do with the aging of the garment... When washed, if the temperature is hotter than the initial treatment... or if you have a "Helpful roomie/siggy other" who comes along and washes it for you at the hottest temperature thinking it's no problem, it will shrink because it's not used to being washed at that temperature.
the only reason you need to wash denim more than twice I would say, would be to remove overdye/oversaturated dye. Where the denim has been dyed so much that the dye might come off on your fingers, or when in the wash, could dye other parts of the costume or clothing you attach it to darker colors or blue, depending on the denim color.
Hope this helps
Katsucon 2014, Otakon 2014