I agree with what has already been posted. Unless you plan on doing a lot of quilts there really is no need to buy a lot of the big cones of thread, at least not right away. ;) I too have bought all the sets of thread CT makes. I hate the invisible thread (which is nylon or polyester) and after using it in a class to make a couple of blocks I gave it away and vowed never to use it again. Instead I match my thread when appliquing and use it for quilting. I do notice it tends to be a bit linty with my machine but since I clean it out frequently I don't worry about it. One spool will cover the quilting for a queen sized quilt. I don't normally do utility style quilting, which is straight lines or simply enough to keep everything down. I like doing designs and things like feathers, which take a bit more thread. I've never run out of thread when starting a project with a new spool and I haven't used one color enough to need to buy it in the cone size. Of course, this is the year for me to get a bunch of UFOs done so I may change my mind about that before the end of the year.
I use only 100% cotton when piecing and mostly when doing applique and for the exact reason above, the man-made threads are so much stronger than the cotton fabric that over time it will rub and cut the fabric. Now this could take 5 years or 30, but either way I don't want to have to redo a quilt later on because I didn't use the proper thread to begin with. This used to be very true of older threads but I'm told the new threads, such as the ones put out by Superior, aren't supposed to do this to our fabrics. Still, I'd rather not learn the hard way that it isn't true so I just stick to the 100% cotton thread as much as possible.