I have found that I really like Bottom Line thread for the bobbin and for hand appliqué. And I also really like the bobbin thread sold at Jo-Ann's (can't remember the brand name right now but it is a very well-known brand). Sometimes it's hard to find, because they only carry it in white and black, and the displays of thread are vast. But it is polyester and nice and thin, which helps with the seam allowance. Why? Because you have less thread thickness to press the seam allowance over.
After experiencing greater accuracy in block sizes with special bobbin thread, it's just something I buy now.