Always single here.
In fact, I've never seen it done with a doubled thread, even on the many, many antique quilts I have or have seen.
If you have doubled your thread for strength, perhaps you'd like the threads made particularly for hand quilting such as Mettler 40 wt. cotton or YLI Hand quilting thread. Many brands make a "quilting" thread.
These threads are generally a slightly heavier/thicker weight; sometimes they have a bit of sheen, or stiffner, or 'waxed' feel to them. When I use a thread that does not have this, I run along the cut thread with a little round of beeswax which I buy at Joannes. There is a substance called Thread Heaven, which comes in the tiniest little square navy blue box, which serves this same purpose.
At shows you'll see a high quality of hand quilting, but generally there's no right or wrong way to quilt your own work!