Well, I came from one of those mothers who still haunts me if I don't trim my threads...
However, in a class I took one time the teacher who was an award winner said to always trim your threads. She has seen judging on two different quilts come down to an un-cut thread being seen through the front or back of the quilt.
I don't think I will ever get to the point of being judged, but I do try to make them as close as possible.
Happy Quilting,
Susan