Is there a reason to use both top or both bottom screws on a standard outlet?

Question

I have a receptacle that was wired with the black on the top right side screw and the white wire on the bottom left screw. Why didn’t they just use either the two top screws or the bottom two screws? I want to add a wire off of this receptacle to add another receptacle in the series.

Answer 1

Why didn’t they just use either the two top screws or the bottom two screws?

There's no requirement to do so. If the tabs are not broken off, the terminals are equivalent.

I want to add a wire off of this receptacle to add another receptacle in the series.

Then use the other two screws.

Note that you're not required to wire outlets in a literal string. You can "tee" off an intermediate outlet, at which point yes, you would have 3 wires on each side. You handle that either by pigtailing, or by selecting "spec grade" receptacles whose screw-and-clamp method permits 2 wires under each screw, or 4 wires per side.

Answer 2

The why is how they held the outlet in their hand when tightening the screw.

The two screws on each side are connected with a small metal tab between them, so which screw does not matter.

Adding another cable/black and white wire, just use the other screws. Turn the breaker off first.

Newer code requires the use of torque drivers on screws/connectors.

At the time, you can also change which wire is on top or bottom.