Do not use pearl cotton to tie a quilt. We used that as per the instruction of our 4-H leader, and within a few months all the ties were coming loose. I pulled each tie tight again and used Fray-check to keep them tight. I had made a second quilt at the same time and sent it to Kentucky with a group that were going to work at the missions. I can't get it out of my mind that I sent something that was going to fall apart in a short time. These people have so few nice items, I had hoped I was sending something that would be useful and would last.
I usually tie with yarn. If you use wool yarn, the more the quilt is washed, the tighter the tie gets.