Not as easy as that. I live in Ontario where most food items have no tax, some items have only the provincial portion, some items have provincial and federal portion. Some items you think should be tax exempt aren't (granola is tax exempt but granola bars aren't). Anyone who gets it within $10.00 anywhere has done a really good job.
I did the calculator thing when I was in school and pretty broke. The way we did it is that if in doubt assume the higher amount, so we never were embarrassed at the counter being over. That being said I can understand if someone is REALLY bad off that wouldn't work as they wouldn't be able to buy everything they really needed. In my case it worked because although I was a broke student I was following that strict budget because I didn't want to be a burden on mom and Dad but if there had ever been something that I really needed, a medication I couldn't afford when I was sick for example, well Mom and Dad would have been very upset with me if they found out I didn't call and ask for help.
I'm lucky now that I have enough money that I don't have to worry about budgeting to within X at a store.
My pet peeve at stores for customers are people that get up to the cash register without having started to look for their method of payment. If you are off by a bit or something like that and have to find a second payment ok, but if your paying by card don't go searching your bag for the card after the cashier already rung everything up and told you total.