Hello from the Pot Holder Queen of the 1980's. My Mother in-law still uses her pot holders. All other's have been sent farther away so I have no idea as to the wear. I always make and give away. I have none of my own.
First. In the 80's I used thick towels, mitering was hard to do and rounded edges was neccessary. Now the new batting for potholders that will not burn hands ( remember the fluffy craft batting does not help from heat at oven temperatures) is less stiff.
I don' think there is a right or wrong. I go with "I do it the same as a quilt." I miter them. But, then that is up to the roundness of the corners.
Good luck,
Ell