For piecing I keep a lot of cream colored thread on hand (usually buy it 50% off at Joann's). It a good neutral color that even works well with dark fabrics. I probably have 3-4 big spools of this always. I haven't done a whole lot of machine quilting, but when I do I just buy the color I need to match at that time, this has given me a small collection of colors to have handy. But for piecing I really only use the cream, so I make sure to have a lot of it.
Cheers!
Rachel