The caveat for Sam Watkins' memoirs is the same as the one for Mary Chesnut. He first published them in 1882, so his mind and memory had 20 years to work out the details and fill in any gaps. At least "Company Aytch" is presented as a memoir and not a contemporaneous diary, as Chesnut's is.
That said, if Watkins was going to whitewash his story, he might have left out the part about setting the negroes free and focused on the invaders from the north angle. So it seems probable to me that it was part of his thinking in 1861.