We got a letter from insurance saying we had gotten $50 each (total $100) in rewards. We filed our taxes last week (1/27). Do we need to file an amended return or wait?

No, you do not need to amend. The IRS considers these account rewards to be rebates or discounts on purchases, and not taxable income. 

I thought we might. The letter stated that it was considered taxable, but I wasn't sure with the amount, if it would be or not. I didn't want it to cause a delay or for our return to be rejected.

If you received a letter with the reward saying it is taxable, then yes, it needs to be included on your return. 

Do not amend until your original return has been fully processed to avoid unnecessary delays.     

How to amend my return?