I use double thread.

I use machine binding for all my quilts except heirloom ones. The machine binding is much better for children's quilts...

Thanks for all the advice. I'm thinking about trying
machine. I'll let you all know although it may be a while.

I attach the binding with machine but hand bind close stitches with double thread.

I use double thread on all hand sewn binding, and knot it every couple of inches. Single thread, in my opinion, is just too easily broken.