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Josh K.T206's explanation makes the most sense and is what I have always understood with respect to soaking - if you bleach/soak to clean (which ordinary water typically wont do) it will be rejected if discovered (bleaching, of course, will also leave a distinct odor). The slip will also state that the card was soaked/cleaned as the reason for rejection (mine did not).
On the other hand, I think routine soaking in water to remove scrapbook paper and glue is common and normally grades. Finally, I have never heard of removing wrinkles while soaking - the issue was discussed by the brain trust on the psa board and made little sense.