Superior Threads has some great information about threads and needles. The type of thread you are using also plays into which type of needle to use and not just for metallic threads. Try to get the 30 weight quilting thread through a 14 needle and it just doesn't work. I can't get the King Tut through a 16! I also had fewer problems when I was teaching myself to free motion quilt with an 18 titanium.
For piecing with all purpose thread, I use an 11 or 12.