i try to use vertically cut (VC) border strips whenever possible. since using WOF requires piecing the borders, i don't hesitate to piece the VC borders if i don't want to buy a looooooooong piece.
there's nothing wrong with WOF borders. i just get better results with less aggravation if i do them VC.
when i have to use WOF, i starch the bejeezers out of them before stitching. that helps reduce the chances of stretching them into waves or ruffles. AND a walking foot is immensely helpful when attaching WOF borders.