When I have any sashing, I cut the sashing what the length should be according to what the block measures (or several blocks). Then pin at starting edge and ending edge and fit it in between as I sew. If its quite long, I also pin it evenly across the center section. That will pull up snug any curvy bits.
When sewing long strips it sometimes helps to sew one strip from the top then add the next strip to the bottom edge of that strip and sew in the opposite direction.
Fabric weight can made a difference. That's why most quilters prefer a nice weight cotton with good 'hand' to it so it sews up more evenly and doesn't ruffle. Thinner fabrics sew up differently and can curve.