Marni

I use tan or grey for all my piecing.

lfletcher

Beige or Light Grey for lighter fabrics. Dark grey for darker fabrics. If it's light and dark, I usually use a medium grey. I piece entirely with Aurafil. Love it.

nycquilter

mostly white, too. though if I'm sewing black or dark fabrics, I turn to black.

susie-susie-susie

I piece with off white or grey. I rarely use white except on white fabrics and rarely use black, only on black fabrics or very dark fabrics. The thread doesn't show and using off white allows me to load many bobbins and that saves time.
Sue

mighty

Taupe, gray, and black or navy.

omaluvs2quilt

Mostly grey, but occasionally natural/beige for piecing.

paulina

I use whatever color I am going to quilt it with.

mosquitosewgirl

I almost always try to use a coordinating color. If I don't have one consistent coordinating color, then I use a neutral, but only white when the fabric is predominantly white.

CarrieC

I use white for white fabric. Unless I am using thread color for drama or accent I use mainly taupe/cream/beige and grey. I've found I really like a varigated grey thread.