As an American, I have always written both the tip and the total on the slip. Until this moment, I have never actually considered only writing one or the other.

I probably will continue to write both as I'm a little paranoid about people making changes to the values before they get put into the machine, and having to change 2 lines is harder than 1.

Is that more likely than you making a mistake and the person picking the higher value?

All of the americans i was with told me to do both. But they were drunk too, so who knows.