If my batting would permit it, I would tie at the center of each pinwheel and in all four corners of the blocks. For the sashing, I would do the center of each cornerstone. If I needed more in the sashing, I would try for the center of each rectangle.
I used perl cotton on the quilt I tied and it was easy to work with. I cut them long so I could trim them a bit after I washed the quilt. There really was much less fraying on the ties than I expected.
I love the quilt. I'm a sucker for all the "antiques."