Agreed by everyone. Along with 3-1 and 4-2, one of the three point-making openers that need no thought.
13/7 8/7Every source agrees
The position after 13/7 8/7. The 7-point is highlighted.
Why this play
The bar point, your 7-point, sits directly in front of your opponent's back checkers. With the 8-point and 6-point already yours, making it gives you three consecutive points.
Three in a row matters more than three scattered. A checker trying to escape from the 24-point now needs to jump a three-point wall, and each point you add makes the jump harder.
It is completely safe. No blot is left anywhere on the board.
The mistake to avoid
13/7 13/12 runs a builder down and leaves a blot instead of making the point in front of it. Always make the bar point.
How settled is this? Every source agrees. There is no serious argument against it.
Knowing the play and making it under pressure are different skills
Play a rated game and the computer reviews every decision you made afterwards, including this one. The dice are sealed before you move and verifiable after, so what the review measures is you.