I stitch in the ditch around the motif I want to stand out. Like a star made of 1/2 square triangles. I don't stitch in the ditch on seams that are part of the 'background'. Then FMQ in the design elements of the block. This kind of quilting is gorgeous because it highlights the design elements of the quilt very specifically. It is also very time consuming. Here's a photo:
Block: quilting by
kristijoy108, on Flickr
If I'm doing an overall design then I would just FMQ over everything and skip the 'ditch' part. Here's a photo of two blocks done like this (quilt-as-you-go):
Vice Versa BOM - May Block (City Lights) by
kristijoy108, on Flickr