Candace, I am with you; use what works! I use cotton only for piecing. But when it comes to machine quilting, recently I have been using American-Efird poly wrap. Then again, my thread choice depends upon that all important test run on my sandwiched quilt. I quilted one piece that would not take anything but C&C and it was quilted on the same machine as my others.
Another big point to watch is using the proper needle. Sharps are for piecing. For machine quilting I use either a top stitch needle or the regular quilting needle.
Bev, there are lots of things that make a quilt turn out properly; perhaps asking your local Guild membership to help you or going to the library and getting a quilting book might help. While I have sewn for over 55 years, I have been quilting under five year and I have so-o-o-o much to learn! I am thankful for this Board. Good Luck, and I am sure a well-informed sales clerk at your fabric shop can help too.