For me it would depend on the quilt, piecing, backing and what I plan on doing with the quilting. Sometimes I match, sometimes I blend, sometimes I go for the gold and make it stand out!
I treat the backing like a quilt in/of itself. So my bobbin thread will depend on what the backing is.
One quilt (
Kodachrome) I did in all black/white fabrics with a solid black backing. I used bright lime green for the top thread and white for the bobbin. It is a very striking quilt.
One quilt (Bubblegum, which is waiting for binding) Is a bali pop in bright pinks with a leopard print in lime green for the backing. I used a variegated green for top thread and a variegated pink for the backing
One quilt (
windows to the world) I used a batik charm packs with black solid backing. I used a silver/black variegated on the front and black on the backing. I wanted the top to look like water cascading off stained glass. So I wanted the variation in color for the quilting.
And one quilt on rack ready to be quilted (Morning Coffee) is another bali pop in browns with brown backing, I am using a variegated brown for both top/bobbin, so that the quilting blends.
Each quilt "tells" me what it wants for quilting. I will say though I tend more towards the dramatic. But I do treat the backing as a "whole cloth" type quilt, and usually pick the bobbin thread based on what I want to see on the back.