The Petroff is my repertoire against 1.e4 2.Nf3, but I just got crushed in an ICC game in the 5-minute pool in the main line, so time to take another look.
1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nf3 Nxe4 5.d4
Why do I consider this the main line rather than 3.d4 ? Because it transposes, 3...Nxe4 4.Nxe5 d6 5.Nf3, and 3.Nxe5 is more forcing. Playing a less forcing line allows the opponent to deviate, so it's a purely practical consideration to break a "tie" in what is theoretically exact equivalence. Ahh, you say, but maybe it is White who intends to deviate, with say 3.d4 Nxe4 4.Bd3. But White gets no clear advantage after 4...d5 5.Nxe5 Nd7 6.Nxd7 Bxd7 7.O-O Qh4 8.Nc3 Nxc3 9.bxc3 Be6, needing to weaken the king position with g3 if she wants to eject the black queen; and black is threatening to force that anyway via Bd6. Maybe more research can make 3.d4 the the main line (again), but it is tbd in my opinion.
Now I consider the main line to continue 5...Be7 6.Bd3 d5. Equivalent to 5...d5 6.Bd3 Be7 of course, but this time the "tie" broken by the non-necessity of protecting the N on e4 until after it is attacked. And I consider 5...d5 6.Bd3 Bd6 an inferior line. In the game I lost I played 5...d5 6.Bd3 Nc6 here instead and soon went far astray after 7.O-O Bd6 8.Re1 Bf5 9.Nbd2 Qe7 10.Ng5 Qxg5?.