While you may get away with it from time to time, my failure to produce a check list or at least mark the instruction sheet with a pen resulted in me failing a level 2 cert flight yesterday almost certainly due to the lack of the washer.
I Was flying a Wildman Journey 75 with a AT DMS 38 J270W-14A. My calculated (optimistic) altitude with just under 5000' with an optimal delay of 13.1 Seconds. My primary method was an Eggtimer apogee with the motor intended to be secondary. But with the tolerance of delay grains I fully knew that the motor charge may fire first.
I was using the included instructions but not marking them. When I went to pull out the washer I removed the igniter from the bag first and the covering tube fell off and into the snow. As I wanted to avoid contamination I looked around and decided to just to empty my pre-weighed charge which was in a centrifuge tube like they supply and put the igniter tip in the tube to protect it.
Well because I wasn't marking off items on the instruction sheet, the distraction resulted in me missing adding the washer first.
My flight was perfect on the powered section but the rocket did go ballistic and became a lawn dart.
After reviewing the video I took you can see that the motor charge went off at the early side of tolerances and the puff is consistent with the charge energy exiting through the back of the rocket.
While hard to see the arrow is pointing at the in tolerance but early cloud from the event. What is hard to tell from that image is that it is 1000's of feet above the exhaust plume just below it.
I am not sure what the failure rate is without the washer, but as they sold the DMS motors for a while before adding it, you may get away with it....until you don't.
Due to some experimentation I had well over a dozen test charges with the load I used and once I developed the initial load and added 20% I never had an in flight or ground testing failure and I was using to sheer pins with a fairly loose fit that makes other explanations for the failure to deploy unlikely.
Still 100% my own fault for not taking steps to ensure distractions didn't cause me to miss some steps.
But as there is fairly ambiguous replies to this question on here I wanted to share what is a pretty reliable example of failure due to not using the washer on a DMS motor.