It's a huge thing with judges in quilt shows. Doesn't matter if it's a local guild show or Houston, but they'll all make a note on the comment sheet if you DON'T sew the miters down.
Personally, sometimes I do and sometimes I don't. Some of my older quilts (when I didn't know it was the "thing" to do) have been washed a gazillion times and the corners NEVER came undone. Maybe it's some holdover from when everything was hand sewn and the miter wasn't as strong as a machine-sewn one.