We stock Skirting less than 150mm high. If they are higher than that we tend to mill them to order, which is done in our machine shop and generally takes a few days. The reason for this is because freshly machined ensures the quality and flatness in the boards, so reducing the risk of cupping or warping.
Another way we help them not distort is putting relief grooves in the back. The photo on the left is WRP Southern Yellow pine with relief grooves in back, which will also assist if you are gluing to the wall, and the photo on the right is an old sample from a customer that has cupped. Cupping can also be caused by a damp environment such as recently plastered walls, as water is in the air and will be picked up by the timber. Please ensure when you receive the timber it is kept in a dry place.