It is true that you can sometimes get Canadian quarters through in the US (it is easier with pennies and nickels, as they are harder to tell apart), but a merchant can refuse them - and if you show up with a lot of them, it’s more likely that they will notice. If you have a bunch of them go back to the person and tell them you want 25% more to make up for the difference!
Canada has eliminated the one-cent coin (your Canadian sales receipt will show a rounding-up or rounding-down to the nearest nickel). So shouldn’t have to worry much longer in the U.S. about being handed a Canadian “penny!”
Here in the Detroit area, where there are lots of people going back and forth between Canada & the US, it is not unusual to get an occasional Canadian coin among the change you get at a store. Sometimes a store cashier will balk at taking Canadian coins, but usually they are accepted at face value.