I don't know how big your patches are but keep in mind the recommended distance between quilting when deciding where to put your ties. Personally I am not a fan of using yarn as I think it doesn't stay nice over time. I prefer to use perle cotton or crochet thread. As you know, there are several weights of crochet thread so lots of options there on how many strands to use.
I recently watched a Donna Jordan tutorial that she demonstrated tying a quilt. After making the knot, she put a drop of FrayCheck on top of it to reduce the chances of it coming undone. That was a new one for me and something to consider.