Ideally you'd have the recommended 1/48 slope (1/4" per foot) in the line. The line is probably a 4" CI (?), so if it's sloped a little bit backward, then you'll have a little bit of a puddle left at the lower end when things settle down after a flush, but not likely any waste to speak of. If it's "always worked" I wouldn't bother fixing it. A real plumbing pro might have a different opinion.