Wow this is an oldie but goody that resurfaced. Love to Sew, It all depends. On this particular quilt I marked the main motifs before loading. I wished I had marked the cross hatch too as I got off on a few places.
For the most part, if the motif I want is large and spans over the entire quilt surface I mark before I load. I will also mark before loading if the border needs to be divided up into equal increments like for a switch back feather.
If I am using the piecing for my quilting motifs, I will mark as I go.
So in a nutshell, big area that impacts the overall quilt, mark before loading. Little areas of quilting where I can use piecing seams as reference points, I mark as I go.
For anyone interested here is a link to this quilt at the halfway point
Longarmed wholecloth part 2 half way there I can't find the final post. The QB advanced search function hasn't worked in several weeks so I can't find it.