Credit card rewards are not taxable because they are considered to be partial refund of the amount charged. However, if you use the card for business expenses and you (or your company) deduct the full amount of the charges from taxable income there may be an issue which you should discuss with an accountant.

Rewards earned through banking (e.g. for opening an account or maintaining a balance) may be treated differently, but again you should discuss your specific circumstances with a professional and not rely on Flyertalk for decision making.