Actually, it is said that the true history is revealed by the Rio Ponelyph alone. According to One Piece's wikia page:
According to Robin, the Rio Poneglyph (...) is the message carried by all the poneglyphs along the Grand Line. It contains the true history of the One Piece world (including the "Void Century" of which research is forbidden). In order to find it, one must take all other passages from the other Poneglyphs with them on their travels, and it is only completed upon reaching the end of the world; in other words, the island Raftel.
Therefore, we can argue that all the Poneglyphs are required to reach Raftel in the first place. However, this is probably not true since none of the Poneglyphs so far even talked about Raftel. Things may change as the crew gets closer and closer to the end of Grandline.
Also it is important to note that in her search of the void history, Robin is not starting from scratch. In her flashback during the Enies Lobby arc, professor Clover shares his hypothesis about the void history to the Gorosei ("Five elder stars" in Japanese) while Robin is listening. He and the other archaeologists from Ohara had already uncovered much and seemed close to the truth according to the Gorosei's reaction upon hearing it.