Michel,
Well, I have companies registered in a few places including the UK and USA. The initial advice was expensive, but maintenance is easy. I have no experience of the Canadian system but I feel sure that the structure of a Canadian company maintaining servers in Canada, billing in Canada and fully resident in Canada is what exposes you to sales tax.
As for tax evasion- sales tax is different from income tax as it derives from local consumption, not revenue. It is not illegal to specifically export service... you will still have to pay income tax on the $ you repatriate to Canada. You do need advice, even with tax treaties in place you can be exposed to double-taxation unless your structure properly reflects your intentions.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
