let's face it. to cut your border strips along the selvage means you'll have to buy a lot more of that fabric. IF you can afford it AND IF you know you'll put the leftovers to good use, then ... sure ... cut lengthwise.
but, if none of those apply, then cut across the width and piece your border. pin every half inch. sew slowly and carefully. resist the natural urge to pull on the fabrics as they feed into or out from under the foot. just use your hands to guide the fabric - not to push or pull.
if it starts to look like it isn't feeding straight and even, stop with the needle down, lift the foot, straighten it out, then start again.
i have just this minute remembered something our home-ec teacher used to make us do. it's called "staystitching" and is often used to stabilize curved pieces in dressmaking. before you sew the border strips to the top, run them through the machine first by themselves. again - remember to simply guide. don't push or pull. sew a leeeeetle bit less than 1/4" from the edge. i'll bet that would reduce or eliminate the ruffles we sometimes get when the WOF strips stretch during assembly.