I use crochet thread or embroidery floss for my tied quilts. It is important that the knots are very secure.
If you use batting, the ties should be the distance apart that is recommend on the batting package. When close ties are recommended, it is because the batting is not needled or scrimmed and needs them close, so that it hangs together over time. If you don't use batting, then the ties are there only to keep the layers of the blanket together and prevent twisting and wadding up. They don't have to be as close.
Enjoy your project!
Dayle