Is There Any Reason I Should Not Wash This Stamp?

No. Just soak it in lukewarm water. If you're trying to clean off any of the cancel or any other markings, it probably won't help.

Why would you want to wash it?

If there is gum/gum remains and paper, I routinely soak them in warm water...taking care to remove whatever is on the back. Gum, especially with saliva and paper, or hinges are a breeding ground for mold.

I wonder what the cancel was. Looks like "OA" in the blob.

Thanks for the opinions, guys. The 'OA' (?) is why I ask. The cancel ink looks different than what I'm used to and I'm curious to see if the stamp will clean up at all. Off to the wash!

I would not be too quick to wash that
The negative OA is part of a cancellation, and an unusual one at that.

Why take an irreversible action on something you do not quite understand? To me it seems more prudent to act AFTER you know what to expect, and not before when the alteration could destroy the very thing you are trying to learn about.

While it is true that normally cancellation ink is not water soluble, odd things sometimes happened and soluble writers inks were used. Don't know if that was done here, but it sure looks like a candidate. What harm does it do to suspend "gratification" and give it a bit more study. I should think we would all prefer to avoid finding out the hard way. Try to solve the mystery of the cancel with out "washing it" and possibly make the stamp more interesting when it is understood.

Uh oh.

I soak almost all of my used stamps if they have hinges or old gum etc. It uncovers alot of hidden faults on the back but I would never even consider using any kind of chemical to clean a cancel or change the color of a stamp.