This is a tip I learned in a class many years ago and it's still one of my favorites. I use it a lot when making quilts and I use it every time I sew binding strips together.
I could avoid this problem if I took the time to draw a pencil line diagonally across the binding to give me a stitching guide . . .
. . . but that always seemed like too much work! Besides, I kept thinking each time I could get the angle line correct - that this time would be different. Ha!
Then, one day my binding sewing changed forever! An instructor came to class with her machine that had this strip of painter's tape on it. She said she loved this tip so much she always shares it and even brought a roll of tape that we could all use right then and there.
We had to find exactly where our center seam line was on the machine. To do this, put the needle down and lay a piece of paper against the needle. Square it up with something on the machine bed (I used the ruler across the extension tray on my machine). Lightly mark this center line with a pencil.
Keep that line going all the way to the front (or as far as you like it).