Let me start out by making it clear that I'm not qualified to give tax advice. Nor would I get tax advice from this forum.

I don't see any tax or ethical issues from claiming 100% reimbursement of your expenses. The cash you receive from the cc company is not technically "cash back" on your purchases, and it certainly is not coming in the form of a discount or rebate from the vendors (hotels, restaurants, whatever). If you retain receipts where required by your employee, I don't see any tax issues regarding your claiming 100%.

Now, as to whether the "cash back" itself needs to be declared as income... well, I've never declared it.