I use whatever scrap I have that fits the size of runner/topper I am making lol.
I have recently in the past couple of weeks completed six runners and toppers from as small as 8x12 to as long as 12X67 and used some cotton batting in some, warm and natural in some and fusible in fleece in some..all got washed and dried and all are laying nice and straight with candles, lamps, Christmas decorations or picture frames on them and they are all flat. I should mention that all were quilted fairly dense and I believe this helps in quilts laying flat.