All the joints you make should be joined by a mitered joint but the 6" long pieces is fine if you want to use up scaps.

I have done this when I wanted to use up the scraps it is OK straight across if you are alternating the colours. However, I think it looks more attractive if you slant the join as it is less noticeable.

If you are binding a small item (like a Pot Holder) you can get by with a straight across seam, but if you are doing a quilt if you do straight across, eventually it will pucker and you have a mess. A diagonail seam and if possible make your binding strip diagonal across the fabric. If you start corner to corner on a 36 inch piece of fabric, you can get enough binding for a small quilt, like a lap quilt or if carefully cut you might be able to do a twin size quilt.

When I bind a quilt that is going to be featured in a magazine I certainly sew my binding strips together at an angle - it's easy to do and when you sew together that last section of binding you will never, ever see where you started and where you finished. But, if I am doing one just for myself and I have scaps I want to use I will but ends together just naking sure that no seam ends up at the corners.

Isew the binding on an angle even for a pot holder.