I agree with those who say it is not safe.
Sure, you can use it that way, and will probably get away with it. I agree that the risk related to having the arbor nut one thread too short is low. At the same time, we should also recognize that consequences of a failure could be most severe, even life threatening. Spinning dado blades on a radial arm saw have enough inherent risk without adding to it.
I know the risks, because my first woodworking experiences were on my college roommate's radial arm saw. We did a lot of dados on it while remodeling a kitchen. Every cabinet door had four dadoed lap joints in the corners, and there were lots of them to build. (As an aside, I am not saying that it is the best way to build cabinet doors; I was just following my roommate's lead and that is how he wanted to build them.)
With that in mind, I would take the time to get it right. I would buy the correct flat washer and use it to replace the standard thicker outer washer. Good quality balanced flat washers, intended for dado sets, are available from table saw manufacturers, among others. Search for "blade stabilizers", but I don't know how thick they are. Unfortunately, the blade stabilizers that I found do not include a thickness measurement. They look like a great solution if they are not too thick.