"I'm using minimal torque to hopefully prevent damage to the casing."
I am a little concerned about this. The back cover is good quality steel and is intended to be torqued to spec. I don't think you can damage the back cover by omitting the washers. Six of the studs are long and go all the way through the rear aluminum case. Under torquing could allow the back cover and/or rear case to shift or flex.
Thanks for the heads up. I'll re-evaluate this for the back cover.