They should be able to tell you the taxes in advance. Check with SPG or the hotel. No point risking it, though I seriously doubt that they will charge you anything except the $3.50 in city tax (if that).
I asked would I have to pay any taxes and if so how much and was told:
"Your free night for your below approved Best Rate Guarantee is a free night and
the taxes are already included for you. You will be responsible for any incidental charges while at the hotel."
Based on what I asked this is a pretty clear "no taxes are due". That said if they charge me the $3.50 I'm hardly going to complain.