Are we sure they used it overhand? I mean really sure?

[Shigawire]

The famous Chigi vase, posted by Piddyx, dates from 650 BC - depicts a scene of the Messenian war of 800s BC, between Spartans and Messenians.

It's one of the more famous depictions of overhand.

I think Watchman's points are very valid, in that they would use whatever method suited the situation. For cavalry, underhand might just suffice and be the best method. The same would likely be true in fighting relatively unarmored Persians. But in tight hoplite vs hoplite fighting, overhand is the only way to wield the spear effectively. There's also the issue of how armored the opponent is. With a bell-cuirass, hoplon and corinthian helmet, you basically have one viable anatomical target; the neck and face, which provides further argument for using overhand.