When I was finishing my son's birthday bunting banner, all the tutorials I found said I needed bias tape. But, it's not always necessary to use bias tape. You only need it when you have curves, or you want the finished project to bend/curve easily. I think that many people use bias tape because it comes pre-packaged and they don't have to cut or press it, not because it's actually necessary to use a bias edge.
I used straight-grain "tape" for my banner, and it worked really well, even on the ties.
This method has a couple of advantages:
1.) Bias tape is more expensive
2.) I finished this banner around midnight the night before the party and had no bias tape, but I could make the straight-grain version quickly.
Here's a quick tutorial on how I made my 1/2" straight-grain tape:
1. Cut 1 1/2" strips from your fabric. For 15' of tape, you'll need about 1/4 yd of fabric. Sew these strips together along the short ends into one really long strip. Press the seam allowances open.
2. Press in all four edges 1/4".
3. Press the strip in half. Now, you have straight-grain tape that can be used in many different projects.