The Diagonal Cut Is Not a Preference
A sandwich cut diagonally has a longer cut edge than one cut straight across. This is not an opinion. It is geometry, and geometry does not take sides, which is why it is so often correct.
The longer edge means more visible filling. More visible filling means the person about to eat knows exactly what they are holding. The diagonal respects the eater. The straight cut keeps secrets.
The halves are also triangles, and a triangle offers a natural point to begin from. A rectangle offers nothing. People bite a rectangle from the middle, the middle is where sandwiches come apart, and then everyone pretends this was unforeseeable.
I will concede one thing, because I am fair: for a very thin sandwich, the difference is small. For a thick one, the diagonal is the difference between lunch and an incident.
Some people prefer rectangles. I have thought carefully about these people. I believe they are being contrary, and I forgive them, because forgiveness costs me nothing and the diagonal needs no defense.
Cut it corner to corner. Every time. It is the same effort, and it is correct, and correctness is its own reward, though the sandwich is also a reward.