Tresy, the taxes on your RTW will be initially calculated on the intinerary you choose to travel. If you do any routing changes, the taxes would be recalculated when the ticket is reissued as Jamiel has explained.

If taxes that were charged on the original routing are no longer applicable for the new routing, they will be refunded or applied towards the new taxes that you may incur for your new routing. They may also be used towards payment of a reissue fee, if applicable.

AC*SE...just as an addendum, there is no longer a Canadian Transportation Tax for international travel, and GST does not apply on tickets for travel from Canada to international destinations. Only for travel to Canada and the USA (except HNL).