This Montreal Gazette article describes Quebec's plans to raise the sales tax to 11%, which combined with the federal GST would raise the total sales tax to 16%. That would end up being 3% higher than neighbouring Ontario and New Brunswick. That's not a huge difference, but one could imagine that for the Ottawa Gatineau region that sales would increase slightly on the Ontario side. Also it would add even more impetus to cross-border shopping, although the Canadian dollar being in the toilet likely cuts that down currently.
The 16% total would be 11% higher than the 5% GST charged in Alberta. At that point it might make sense in Quebec to try and order from Alberta stores online, especially for expensive small items.
Personally one reason that I've never been a huge fan of sales taxes is that they inexorably creep upward, until you are past 20% as in a lot of Europe.