Before getting too hopped up over "wrongful dishonor" here, consider another angle. How does your bank know that the person who endorsed the checks has the authority to do so for the check cashing business? Or that the person bringing them has the authority to present them for payment? You don't.
So, I think your branch manager is on the right track suggesting that the checks should be presented through the normal routine of in-clearing via the business's own bank. At least that way you have that bank's warranty as to proper negotiation.
The better string for you to take a look at is the one on "
cash back for business accounts." It has lots of arguments that apply equally to the check cashing for business question.