I use the "bin is full to overflowing" method of determining when to start scrap piecing. I have some bins devoted to common block units, for example: I have 4.5" HST's, 2.5" HST's, strip blocks, and bins of 2.5", 1.5", 3" strips. I sew the block units primarily when I just have to sew something but lack the discipline and focus to work at length on a specific project. The scraps are winning the war, but I occasionally win a battle!