I just finished a couple of quilts with straight line stitching, and used 11 stitches per inch. With more curvy designs, I'd go up to 13 maybe, but rarely go any higher. With micro-stippling, maybe up to 15 or 16.
In a Judy Madsen class (who does a lot of straight lines in her designs), I asked her what she used and she said 16. I just don't like the looks of that (with my machine -- HandiQuilter) for straight lines.