When I piece I use Superior Thread's 50 wt SoFine in the top and Superior 60 wt Bottomline in the bobbin. Both are polyester threads and I prefer the polyester because of the lint build up from cotton as previously mentioned. I find this combination for me results in a more accurate scant 1/4" seam. The other bonus is that the finer 60 wt thread loads more thread onto the bobbin, so fewer bobbin changes, both in piecing and in quilting. In quilting there is less. Thread build up on the back and as Prism99 mentioned, less than perfect stitching blends in. However, I prefer to match my top and bobbin thread colours when quilting, so if I don't have a matching 60 wt thread on hand, I'll use the exact same thread top and bobbin, and I mostly quilt with Glide 40 wt on top. Again, it all comes down to personal preference for the look you're going for on your quilt, what's available to you and budget, as they are not all priced the same more readily available depending where you live and or shop.