I find starching heavily helps with the stretching of fabrics as well as fraying. As for sashings and borders, I measure from both sides as well as the middle and take the average of the 3. This seems to keep the borders from wavy when I go to quilt it. I'm sure there's more ideas coming your way. I only have 1 UFO panel quilt and I designed it myself to go around the panel. I'm still trying to figure out how to get the sashings I'm adding between each row of blocks going around the panel but I think I know what I can do after hesitating for a mere 5-6 years.