I do use whatever. I stll have thread from my DM & DS from over twenty years ago that I am still using up to piece my quilts. It's kind of nice to know some of my mom and sister are in the quilts I make. I mostly make scrappy quilts at this time so it really doesn't matter what color or size.
For special quilts I do like to use Aruifil in a medium gray (#50). It just disappears in most fabric.
peace