You can now manually edit the tax rate for a particular booking. Bookings will default to the tax rate set for your property under Settings > Your Details > Sales Tax, but if you need to change the rate for a certain booking, just click the little pencil icon next to the tax rate on the booking:
That will allow you to enter a new tax rate for that booking only:
Untaxable or taxable extras will behave as before. Taxable extras will contribute to the tax line; untaxable extras will not.