The batting you use will determine how far apart you can safely quilt. Look at the label. Some you can quilt as far as 10", some need to be quilted every 3 or 4.
Also the fiber the batting is made from also has an effect on how soft your quilt is. By soft, I'm assuming that you mean how it drapes? I like wool batting and bamboo is also drapey...
What batting did you use and how far apart was your sitd?