To be fair, there are so few actual "national" advertising campaigns. They are mostly regional anyways. So, if you're running a regional ad, you know what the final price is in that region anyways.
I know you already know this, but the regional variation on taxes in Canada is at the provincial level anyways. There aren't municipal sales taxes in Canada like there are in the US.
I don't see it as a huge problem since most national companies with something to sell have varying regional prices anyways.
I'm a man, but I can change. If I have to. I guess.