I pin about 6" apart, but I'm using the thin cotton batting. The thought of putting glue, spray baste or starch on my quilts just gives me the heebie jeebies. Yuck. But to each, his/her own....
I never have problems with layers shifting when using cotton batting. It's kind of "sticky" and both the cotton top and backing stick to it, whereas a poly batting is much more slippery.