I doubt 'one washer' will compensate or prevent the blades from striking to the front of the shaft. JMO.
Also most times, I use the Rocket Steelhead expandables (a.k.a. Champion Big Game)--as it is, it takes only 2 ~ 3 rotations (without using the washer) to tighten the broadhead into the insert, adding a washer or two will certainly decrease the amount of thread lock and could possibly cause some problems.
Like I'd said earlier, this is just a concern.
I was hoping that the folks at Easton would have done the necessary testing and probably shared the result with us.