I have done several t-shirt quilts and have always done an all over design, like a panto or straight line quilting using regular cotton thread, top and bottom. I don't care for invisible thread for items that gets used, like bed quilts as the thread isn't near as durable. It brings to mind all those cheap comforters/quilts that you can buy at WalMart.....I remember all the invisible threads, broken and poking me....not pleasant memories. I have found that quilting across the designs doesn't take away from the overall look of the quilt.