Depends on the interpretation of the USPS's Domestic Mail Manual P002 2.2, which specifies that
d. U.S. stamps that are mutilated or defaced; cut from stamped envelopes, aerogrammes, or stamped cards; covered or coated in such a manner that canceling or defacing marks cannot be printed onto the stamps; or overprinted with an unauthorized design, message, or other marking
are invalid for postage. Whether or not a stained stamp is "defaced" or bears an "unauthorized marking" is the question.
It will probably pass through the mail just fine -- envelopes can pick up all kinds of weird stains in the mail so the postal workers will most likely give you the benefit of the doubt -- but defaced stamps in general are not valid US postage.