If you intent to have gross revenue great then 30k, then you must register for a GST number and charge GST for your Canadian clients. You don't have to charge GST for your US clients, but you MUST be able to prove they are not Canadian, otherwise CCRA will deem them as canadian and then you'll be liable for taxes you "should" have collected. In other words, be sure to get the billing address of you clients as part of every transaction.
I'm not sure about your provincial taxes. In BC there is no PST (provincial tax) on web hosting for bc residents.
Whatever you do (when contacting the tax people), get it in writing.
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As mrzippy said with his great advice, contact the taxation branch of your provincial government. Anything on WHT is merely "hear say" and is not a valid excuse for messing up your taxes. Failing to contact the local government about it could lead to major headaches down the road so your best to play it safe.