The main thing I'm wondering about with the impending demise of the Canadian penny

In the Canadian federal government's most recent budget it was announced that the penny would soon be phased out, or at least that in the near future they would no longer be in production.  It was suggested that instead of using pennies transactions could be rounded to the nearest five cents when one needs to give change or pay.

My main question is who gets to decide when when the rounding rules are used and when exact change is given?  For instance, can a store opt not to give a customer a penny if they have one in the till and instead round down to the nearest nickel?  Can the customer pay only $0.40 even if the cashier can see that they have the two pennies that would be required to pay the $0.42 bill in full?

Presumably there isn't much that can be done about a customer deciding to pay with cash or electronic means of payment if one means of payment is more advantageous than the other.  But still, shouldn't we at least think about this issue?

I guess the point of all of this is that if hacking/digital heist movies (cf. Office Space) have taught us anything it's that if you skim pennies from millions of transactions you may eventually end up with millions of dollars.  Unfortunately I haven't yet heard how this problem will be resolved, or if it will be resolved, and which protections, if any, will be put in place for consumers and retailers (though I suspect that retailers are in a better position to make better use of these schemes than individual consumers).

Update:  The following quote from a Globe and Mail story doesn't give me much hope that the aforementioned problems will be adequately addressed prior to the demise of the penny.

Ottawa says it won’t be policing consumer-business transactions, but added that “businesses are expected to round prices in a fair, consistent and transparent manner.”