Typically the washer will make it just a bit less 'grainy' feeling when rotating the blade. It does help to smooth it out but you can't often tell the difference unless you have one to test it both ways. It is true that I've had several Manix, Mini Manix, and even Dodos and Yojimbos sent to me for work over the years that were in fact missing a washer either through QC dropping the ball at the factory or the owner of the knives doing it after taking them apart though so its not always even noticed when one is missing. You are not the only one without one in there. What I suspect happens is that folks think they put it on and since many of them are not the PB washers but the ones that are very light weight and affected by static they often times dislodge or fall out without notice. At other times they stick to the inside and are between the blade and the G10 but not on the pivot as they thought they were and they eventually work their way out and fall out without notice.
I'd suggest getting one yes. Spyderco will ship that out to you. They use that washer in several knives so its something they keep stock of and since the knife is from the Golden plant it won't be a problem usually. Give em a call tomorrow and get a new set on order.
STR