Fired heaps of rounds from a double in every configuration
or possible way and I didn't notice the difference and neither
did the herd of animals - I point the gun, fire and they dropped.

I really don't think it makes a damn bit of difference.

When finishing off animals on the ground as I walk around,
I do tend to fire the left barrel and single load which
leaves the right barrel ready if needed in a hurry AND tends to even up the number of shots per barrel.