It depends on the hotel too.
The HI Berlin City West charged me 3 euros on a reward stay. The tax is 5%, but nobody could explain how they calculated that 10000 points converted to 60 euros. I didn't want to waste time arguing and just charged it to my client (actually the tax is not payable for business stays, but technically I wasn't on business).
But other hotels in Berlin have not charged me any tax on reward stays.
Hilton seems to be able to accurately quote taxes that may be due, so I don't know why IHG can't. Not sure how it is with other chains.