Sewnoma

Depends on the quilt. I tend to use variegated thread mostly on kid's quilts, and I often try to make those colorful & interesting on both sides, so I usually use it in top & bobbin. But if the quilt back is plain, I use same weight of non-variegated thread that matches the backing fabric in the bobbin.

IBQUILTIN

I don't think it would matter unless your thread weights are different and cause breakage

Belfrybat

The couple of times I've used variegated thread, I've used a solid in the bobbin.

meyert

I have done both. I love variegated thread I quilt with it alot. Sometimes if I want the thread to blend with the back I will only use the variegated thread on the top