I have heard stories about how super-rigor course load + okay grades > straight As with not so rigor course load. Is this true? Which one is better for graduate school?
Well, the best thing for graduate school is good grades with a rigorous course load.
When you get beyond that, it’s not a black and white question. It depends on where you’re trading off rigor and grades. There’s no point in overloading yourself with classes unnecessarily so that your curriculum looks “rigorous” and getting a middling GPA, for example. Take a reasonable course load (15-18 credits a semester is plenty). At the same time, you don’t want to take too many easy courses; even if you do get a 3.9 and get into graduate school, you’ll be unprepared for the difficulty of grad school - and may have even missed some foundational knowledge you need.
So the answer really is that you need to balance a reasonable level of rigor that you can handle - basic/foundation classes, interesting electives, a few rigorous electives in your major, some cognate/related classes that will round out your study, maybe a graduate-level course or two - and maintain good grades.