I used to tie quilts with an individual knot and tails, but I learned another more efficient way to tie quilts using the "international stitch." Here are a couple of links to check out this stitch. It does not come out, ever.
http://www.quilt.com/QuestionOfTheWeek/1999/1018.html
http://www.creationsbykara.com/2010/...ed-quilt.html/