I've never gone to a restaurant ever where the owner said, "if you pay cash, you don't have to pay the tax"
Ever, and I go to allot of restaurants.
Same with variety stores.
I have noticed that the Chinese run Variety Store owners never ring anything in. But they damn well charge the tax.
Redementary receipt is irrelvant because the till has its own receipt as well. If Revenue Canada audits them, they are expected to produe receipts. Same goes for remitting GST and PST. If they didn't ring it in, you can be assured that the GST and PST never gets remitted. (That's why in some places they don't bother with sales taxes and just go for property taxes. Harder to beat.)
There's an underground economy for sure - usually in Home Renovations more so than anything where it's, "pay me in cash, and I won't charge you the GST". Though, that said, I have a contractor friend (very good friend actually) who now refuses to that because it reduces his income on his books. It's hard enough for him being self employed to get a mortgage, but showing an income of 30 grand when it's actually 130 grand - it becomes impossible to get a mortgage, or a Line of Credit. As a result, he tells people who offer him cash that he doesn't care because he's still charging them the GST. Plus he was audited one time and they were looking at the amount of materials he bought verses what he billed and it didn't add up.
Also, only really stupid guys offer you the chance to pay cash. Usually, they wait for you to ask, "if I pay cash" because they'd be cooked if you worked for the gov't and reported them. If YOU offer to pay cash, they can always claim entrapment.