What is the most optimal way to visit all races' starting territories?

At level 70 you'll have a hard time getting them all. This is as close as you can get:

1. Try to collect all Alliance races directly in Stormwind.
2. Fly from Stormwind down to Booty Bay. There are a lot of people questing in Elwynn, Westfall, Stranglethorn and they're all very close to each others, Alliance and Horde alike (especially in both Stranglethorns).
3. From Booty Bay you can take the ship to Ratchet.
4. From there you can visit the Orcs and Tauren in their starting areas as well as search the Barrens for possible victims, which are quite popular as well.
5. Hearth back to Stormwind and use the Deeprun Tram to Ironforge. From there you'll have to fly up to Tirisfal (Undead) and/or Eversong/Ghostlands (Blood Elves)
6. Visit Hellfire Peninsula from time to time and check for people leveling there. There's a portal back to Stormwind at the entrance, making it easy to travel between both.
7. Same with Dalaran (Northrend). It's also a good idea to have your Hearthstone here, because you have more options to travel around from here (at least at level 70).
8. If it still works: PVP. Join random battlegrounds and poke them there. Rogues and PVP are a good combination and you'll almost always have a few of them with you.