batting in your table runners is a personal choice- some people use batting---very thin---some prefer them to be flat- and simply use a layer of muslin- or a lightweight stablizer. i've made them both ways- and the one i made my mom with warm & natural---was not a good choice- when she washes it- it fluffs up- and is too thick for her liking- vases tend to tip over- a flatter thinner batting would be a much better choice....i suppose it depends on what table you are planning to use it on too. and the amount of seams in it- a batt will smooth the runner when there are alot of seams-
i have alot of lightweight single sided white flannel- i prewashed that and found it works nicely - gives some body- without (puffiness) but it is a personal choice- and the project itself should be part of the decision