The wavy ruffly borders are indeed emphasized with a larger quilt. It becomes very apparent when hanging a quilt like in a show or a wall quilt or if you ever decide to use a LA quilter or quilt on a rack. I have gotten some that were so bad the only way to quilt them was to put strategic tucks in here and there. One was so bad I had to take the border off and remeasure it and redo it (with makers permission of course!) She too was a new quilter and didn't understand the importance of measuring the quilt through the center and cutting the border fabric to those measurements.
One border was 4" longer than it's opposing mate! Every single border measured differently. Here are before and after pictures to emphasize:
Before fixing, sewn by just slapping fabric on and cutting when you reach the end:
[ATTACH=CONFIG]542010[/ATTACH][ATTACH=CONFIG]542011[/ATTACH]
And here it is after I fixed it, not ruffly at all:
[ATTACH=CONFIG]542012[/ATTACH]
Most smaller utility quilts it will never be noticed. And because you don't quilt on a rack, it isn't noticeable when quilting. It really becomes apparent when hanging that the edge is wavy. Even more than when laying flat on a floor.
If you are happy with the way your quilts turn out, don't have issues with wavy or ruffly borders and feel like pinning is a waste of time, by all means go back to doing it the way you were. There are oodles of quilters out there that do it the way you describe with no issues whatsoever. But if you ever decide to make a quilt for a show or have something special you want to send to a longarm quilter then you definitely want to measure the center of your quilt, mark center point on the top and precut to size borders and pin, easing in any fullness.