Since this one was only a month old, I guess I might as well add my thoughts... I don't think that the grade actually matters. I don't really know why anyone would pay attention to it. Yes, I cared about it when I was playing to just pass songs, but if you are a good player, then you play to get an FC on every song, because that gets you the most score. An FC will normally get you at least an S. I think maybe you can get an A on it, if you get a really low accuracy FC. I think that a good player will tend to focus on combo, accuracy, and mods used, when considering if they have done well enough on a song.
That said, I will accept less than an A grade, and use it as a benchmarker, on songs that are so difficult that I am having a very hard time FCing them. Basically it's a way of reminding myself that I still need to work on that song. The same goes for an A, but if I had a great combo on an A, then I'd obviously do the Bs first, if I saw one. I have so many songs at this point that I just improve them as I see them on the list after I am finished with the new songs that I get.