For piecing I mostly just care about how thin the thread is, and not too dark if I'm piecing lights. So I'll use whatever on top & bottom as long as it's about same thickness (or thinness, as the case may be).
For quilting, it really depends on the quilt - what colors are already involved and what my goals are. Often I try to match the bottom thread to the backing color but sometimes I might go for contrast or use a variegated thread; on the top I might either try to match the top color for something like SITD quilting, or I might go with a bold contrast if I'm doing FMQ that I want to have show up. I also tend to use a lot of variegated thread on top in my quilting if I'm doing baby quilts. If I'm going to be doing very dense quilting like for a modern quilt, I usually use my thinnest threads in top & bottom....unless maybe it's an art quilt in which case I might pick a heavier thread.
So I guess my answer is, "it varies, greatly".
