All of my charity quilts are tied. Some from a club here, all baby size, others with my club in OK which were all bed size. I have 2 king size quilts upstairs tied. Some yarn, some floss, some crochet thread.
All of the quilts we have have stood the test of time. I don't think yarn is a good idea. When I have to replace ties, it's usually the yarn ones. I only tie with floss or crochet thread. They both have their pros and cons. I find it's easier to make the knots with crochet thread than floss, but the floss comes in a TON of colors and you can make it match. I use my fist to measure ties and make sure I go just over the minimum quilting requirements for the batting.