I'd say anyone who can do "Through the Fire and Flames" and "Roundabout" on pro guitar CAN do any song. But if someone's voice is high enough for "I Want You Back", it wouldn't be that hard, but how would that same person fare on a Johnny Cash song? Should "I Walk The Line" be in the final tier for that reason? Or both songs? Or neither song?
What's so hard about seeing if you're in the right octave anyway? If the bar is noticeably higher, you sing higher. If you can't tell, turn up the vocal track and/or use the guide pitch in practice mode.