Originally Posted by feline fanatic

SITD is largely based on personal preference. When Harriett Hargrave wrote her first edition of Heirloom machine quilting she recommended SITD as a way of stabilizing the entire quilt then going back to do the fun FMQ detail after the entire quilt had been stabilized by ditching. This was the recommendation for DSM quilting when the machine is not mounted to any sort of rack. With the advent of spray basting and glue basting the SITD step is not always necessary. If the quilt is entered in a large national show the judges do take SITD into consideration and will almost always award more points to a quilt that is ditched. Obviously this does not apply to quilts that have an all over E2E design.
Ditching does help to emphasize pieced blocks but that can be two edged sword. In some cases the ditching can sometimes hide less than perfect points but other times it can emphasize a shortcoming in the piecing. It can cause issues for quilt tops when seams are pressed open instead of to one side. In those cases there really is no ditch and often you are quilting over your piecing threads and may or may not compromise the quilts construction but if it is quilted sufficiently in other places it shouldn't have any detrimental effect. I do know of some LAQ that prefer to get the tops with seams not pressed open. Both Judi Madsen and Margaret Solomon Gunn have had blog postings about it and the possible negative side effects (pressed open seams).
In most cases I do prefer to ditch but not every single seam. As I stated above, it is largely personal preference for most people. When I hand quilted I never ditched but quilted 1/4"on either side of seams. When I quilted on my domestic I was out of the ditch more than in so avoided it. But once I got my LA I definitely ditch most of my own quilts and client quilts when budget allows or if I know the quilt will go into a show.