Usually I strive to have an even density of quilting across the entire quilt. The quilt can look lumpy and saggy if you have very dense quilting in some areas and no quilting in other areas. So I will normally either do tight all over, or loose all over. That's not to say there can't be some variation, but it's better to try and get it fairly even throughout.
And as others have said, the density is dependent on the type of batting you are using. So if the batting says 'quilt no further apart than 3 inches', you wouldn't want to leave a 6" area unquilted. Some battings will not hold up very well under use and washing if there isn't enough quilting.