Love your quilt! What I would do is outline stitch in black each long row from side to side and then the up and down edges of the largest individual blocks (each block has 3 boxes, I mean the largest of each), and lastly, outline stitch the smallest block (the coffee cup blocks), so that it holds up without the batting bunching up inside over the years. By sewing it that way it keeps the focus on your quilt rather than your quilting. It would drive me nuts to change out the threads that often, and black is a unifying color in the quilt anyway. Please post what you end up doing - can't wait to see the finished product, whichever way you go! It's a 'happy' quilt!