Not a very eco-friendly tip, but if you’ve got any spare cards lying around it’s a good way to use them up.
AT HOP cards will let you tag on if they have a positive balance. If your trip is greater than the cost of a $5 card, it can be cheaper to discard it after your trip and get a new one.
So basically, don't tag off and spend that charge equivalent of the fare, on the new card?