We tie quilts at our church for charity and use yarn 99% of the time, prefer sport weight acrylic because it is easier to thread the needle and get through all the thicknesses. We have used pearl cotton for special orders so the thread matches the quilt top. With today's bonded batting, it is not necessary to tie every 4 inches, we go for 6 to 8 and when we have a top made of 10 inche squares, one in the center is enough. Batts are now made so you don't have to hand quilt so close as before or tie so much. I have been using quilts tied 6 or more inches apart for years, washed them and have never had any trouble with fuzzing or breaking.
Remember to use wool yarn for a quilt with a wool batt inside and don't put it in the dryer, can be washed on a gentle cycle but needs to be hung out or laid out to dry.
Carol J.