Mine was shipped with the washers, nut and bearing installed on the arbor. I just removed the nut from the bottom, slid off one washer, and added the slotting cutter to the mix. But hey! Whatever gets the job done, right? I still have all my fingers. 
Absolutely ...
I came across this (and a couple of other pictures like it) on the net, and figured this was the way to go ...
I also thought that the washers were in place to free up the bearing and allow a nice spacer for rotation. 
Oops! I just did the same thing myself! I bought a Whiteside slot cutter and arbor at the local Woodcraft store and also apparently installed it backwards. I simply removed the nut and washer, slid on the slot cutter, and replaced the washer and nut. I didn't have any problems, but I've seen a few pictures around the web that indicates this is incorrect.
It appears the proper placement is to take everything off the arbor, then slide the slot cutter on first, then washer + bearing + washer + nut. This way the bearing is the "lowest" point on the router bit when you're cutting the T-slot.
I got confused because I had just used a template cutting bit on my router in which the bearing was on "top" (because I was following a template that was on top of the plywood I was cutting).
I found another good image on Rockler's website:
