I'd keep it where it looks good to you.
At the shop, we use a different method. We set the machine to do a zig-zag stitch, with dark thread on top and light thread in the bobbin (cotton thread). Then, we want the top thread to just "tick" to the back side. Then when you go to sew a straight stitch it's usually right where it should be.
I do set my tension differently for different applications. That's why they made our tensions adjustable.