A lot of the purists call a tied quilt a coverlet (not a blanket) because it is not truly quilted (not my idea but Gerald E. Roy went on and on about it in a recent issue of Love of Quilting).
Tying it on the top or bottom is personal choice -- I generally tie on the top and make sure that what I am using to tie enhances the look of the coverlet. I used to use yarn, but I can't pull it easily any more so I use pearl cotton which comes in lots of colors.