Interestingly, we have evidence also that hoplites aren't just shown with overhand spears, but you can see an underhand one here:

http://www.urmc.rochester.edu/Steril...es/hoplite.jpg

The cool thing about this one is that it clearly shows an underhand grasp can be shown by an artist when it actually was used (and thus an artistic convention of only using overhand, in order to be more aggressive or aesthetic, can't hold always) - and here it is used against a charging horse, a time when overhand would not be used.

Other overhand shots:
http://www2.unil.ch/iasa/iasa_c_est_...ite_armure.jpg
http://socrates.clarke.edu/hop_1_tm.jpg
http://www.peplums.info/images/30thermo/30a-10.jpg
http://www.utexas.edu/courses/greeks...s2/hoplate.jpg
http://www.sikyon.com/Athens/Archaic/images/vase37.jpg

And I did say that acquired knowledge is a "starting point". Not the end. That would be idiotic.