[jee7s]

It's unlikely to have any substantial impact on resistance. If anything it might slightly (very slightly) increase resistance at the connection. Most of the electrical connection is between the flat surfaces of the terminal and the screw's post. Adding a toothed washer would (technically) increase resistance between the top side of the terminal and the screw head, but even then it would be on the order of milli- or maybe even micro-ohms.

The bigger reason for a toothed washer is to provide a more vibration resistant connection. Those teeth dig in and prevent the screw from rotating if the connection starts to vibrate. Whether that is significant in your mind is up to you. But, I have seen teams do this, particularly on battery terminals.