Not exactly sure but don't all digital downloads already include taxes ? (at least in Europe)
What I'm trying to say is that a digital game that costs 10€ in one of the €-states costs 10€ in every other state no matter what tax rate they have.

For example:
Germany: 10€ = 119% 19% tax = 1,60€ 100% = 8,40€
Luxembourg: 10€ = 115% 15% tax = 1,30€ 100% = 8,70€

So publishers can earn more money from games sold in Luxembourg.

Aren't all European Prices with Tax included?

In NA we never have tax included, unless you are using the "points" system of purchasing products. 

GST on games in Ontario is 13%. It's not too bad all things considered.

As far as I know, they are, yes.

If you are in the US, you get charged your state's Tax no matter what, at least that's what I get out of it. With more states passing laws that require web-sites that do business (in whatever form) in the state to collect sales tax. This is to recoup the sales tax that they are not getting from you doing your business online instead of going to a store. (I know my 3DS charges for Wisconsin's state tax when I buy things. (5-to-5.5%, depending on the county)

If you are going for cheep tax rates I think the certal US states and South Eastern states (with the exception of FL) have the lowest taxes.

http://www.retirementliving.com/taxes-by-state (This may help, I wouldn't go with one that doesn't have sales tax since they may charge for CA's sales tax as default.

I heard Alaska or Oregon are nice places if you want to save the tax. I live in Germany, so I can't verify my claim.

Yeah they don't have sales tax but they may charge for CA's sales tax if your state doesn't have one. I don't live in Alaska, Delaware, Montana, New Hampshire or Oregon so I can't say for certin. (Those are the states with out sales tax) CA's tax is highest with 7.25%

Yeah sure, I was just pointing out that usually digital prices are uniform, so you will never get an odd amount like 20,58€. Instead you will pay 20€ or 19.99€ and the publisher takes a hit.
Here you see the opposite.

13%?! Does it change per region/state?

I tried removing province and also tried putting Alberta (5% sales tax), still showing 13% on purchase.