In the past I've always cut it on the bias, (and that uses a lot of fabric and is a bit difficult to do, compared to cutting long long lengths from the straight grain of fabric).
But I was just watching a (very good) youtube slipcover tutorial (I'll put more details about the video here tomorrow... it's 25 episodes... very helpful)... I noticed the gal on the video cuts her piping on the straight grain. Intriqued, I went asked my friend's father, who is an upholstered if he cuts piping on the bias. He said "What's piping?" (he calls it welting). He said he cuts it straight, unless the pattern calls for bias, such as with a stripe.
So I thought... WOW. I'll try it. The above pic is a closeup of piping cut on the straight. It turns the corners every bit as nicely as the bias piping I used on the bottom cushions. I'm a convert!
Next steps on the slipcover: The back box cushions... I've cut everything, made all the piping, and basted the piping to the front and back of the 2 back cushions. (There's a little hand-sewing step left to finish where the beginning and ending of the piping meet at the back). It's a tad tedious, taping the piping ends together, pressing the end of one fabric piece under and lapping it over the other end by hand. Then going back to the machine to finish basting it on.
I installed zippers on the back zipper panel and sewed the zipper panel to the long box side strip... ready to be sewn into box cushions ... tomorrow.