I love to longarm T-shirt quilts, especially if the customer gives me a little "artistic freedom" with the design. I always use invisible tread. I tend to let each individual t-shirt designs help me decide what it needs. Outlining designs, words, doing different background fills, etc. And some customers just want simple meander and that works too. But I would definitely recommend monofilament thread...my favorite one is Essence by Fil-tec. (And also insist that t-shirts are stabilized!)