Carol,
My sewing mechanics say that you shouldn't really use cotton thread unless you have to , for it causes lots of lint damage to your machine. I've not had good luck with common thread like Coats & Clark - they broke too often.
The cotton thread for cotton garments, my 90 year old grandma says, is a marketing recommendation. She and I both use and recommend poylester - Gutermann or Robison Anton are the best, I've found. In the old days, dyed cotton thread would run and polyester is colorfast due to the polymers which capture the dye and hold it fast. I've made many cotton quilts and used polyester thread. The cotton thread I've used is primarily for hand quilting and so it is heavier.
L