With crazy quilts I always tie them about every 4". I like to use a small ribbon and put it through a large eye needle. I find it a bit tedious but it works perfect for the end results are great. Of course there is not an inner batting and I like a heavy cotton or flannel for the back.
Because the crazy quilt is a foundation block it really doesn't need to be batted it is already quite heavy. Your local library should have some books on crazy quilting that should help you out some. Especially since there is quite a lot about the historical, victorian, past to these beautiful quilts.