Well, just because it was said in the beginning of the serial doesn't mean that it's set in stone, especially because there is a new Hokage leading Konoha. Out of common sense a genin or group of genin wouldn't be sent on an A rank mission because they'd all die.

Hokages appear to be chosen in the way Tsunade was, by the town elders (that group of old folks that asked Jiraya to be Hokage before he suggested Tsunade instead). We can assume that if Naruto was to be chosen that way, he wouldn't necessarily have to be any specific rank first.

At this point in the story, I don't think rank matters at all anymore. Most of the main characters have already transcended the normal order of how Konoha fuctioned. To be honest, I'd be suprised if they ever mentioned rank seriously again. Or if they do another Chuunin exam, it may be like the 2nd Hunter exam in HxH (where it was completely over in one or two chapters)