Bias cut ruffles are easier to handle. The edge does not curl under which often happens w/straight cut and then you have to loosen the stitches and uncurl. Also, the bias edge does not shred which happens w/straight. And if you compare the two - bias v. straight - the bias are much prettier.
When applying to the curtain: measure the entire length of edge of curtain, divide in half and then quarters making w/pins; do the same with the ruffle (divide in half, then quarters); starting at the half point pin, adjust and pin within the space/ruffle allowed; then move to the next quarter for even distribution of the ruffles.
When making the ruffles, I use the quilting thread and a zigzag as suggested above.
Hope this helps.
Suz