i generally use silk thread for bindings, it is fine/thin; strong, i have a good variety of colors, and it disappears. is wonderful to work with (fine, no lint, strong) i can make really tiny stitches with it- i just love to work with silk threads and use them for most of my hand work, appliques, english paper piecing, and bindings.
if i don't have the correct color i will generally turn to a good polyester or aurafil for bindings- cotton thread is just not strong enough (in my opinion) i've had to replace way too many bindings for people where the cotton thread simply gave away with time/use/laundering...i prefer threads that are strong-will last the test of time.
i do use cotton thread for machine piecing often- knowing that the quilting process will help add stability & strength to the finished project