As with everyone else, it depends on the use of the runner for me with respect to the batting.
If i want a thin runner, i do not using batting at all, but in those instances I rarely quilt them.
If I want a quilted looking runner, then I use a thin layer of cotton and/or poly/cotton.
If I want the runner to double as a hot pad on the table, I layer a thin 100% cotton with a layer of insul-bright.
(the stuff for pot holders etc) It is a little thick on the quilting, but it can be used as a hot pad as well.