I was taught to use a single thread. My teacher was my MIL who comes from a long line of Mennonite quilters so I consider her my source. All the ladies at the church quilting bees used single threads as well.
There is a difference between the handquilting thread and the cotton thread you use in your sewing machine. NEVER use handquilting thread in your sewing machine because it is coated with a waxy substance that will gum up your machine. There are a couple brands at some popular stores that are easy to mix the two up. I finally noticed the spool was a different color (but not obviously so).
I always use beeswax (even on quilting thread) to make it easier to handle. I always use a tween needle. Wish those suckers could thread themselves. It seems the eyes just get smaller and smaller. :roll: Pretty soon I'm going to have to add a magnifying glass to my needlenose pliers when I'm handquilting.