Depends. Weekend before last we made the 400 mile trip to our son's house and we made sandwiches to eat in the car. I cut that in half to make it easier to handle while eating while driving.
For lunch yesterday at home, I made an egg salad sandwich and I did not cut it in half, but I should have because a bunch of the egg salad fell out while I was eating.
I always cut my sandwiches in half, not diagonally. I have no idea why, I never thought about it before now. Thinking about it right now, diagonally makes no sense for situations like eating when driving because it makes getting half the sandwich out more difficult because a portion is ALL the way in the sandwich bag when you pull the first half out.