Quiltlady, I'm no expert by any means but as I understand Heirloom quilting the difference is in the techniques used. An heirloom quilt would contain lace shaping, pin tucks, entredeaux etc. To make a long lasting quilt (which I hope mine are) that may be destined to become family heirlooms, I think cotton thread would be just fine.
I think of it this way: Quilts from the 1800's are still around, and our cotton fabrics, batting and threads are made stronger today than they were then. Cotton fabric is more tightly woven by machine, batting doesn't beard the way it used to, and so on. I try to stick to good quality materials and let the rest go. JMHO :D