I've done Irish reenactments of medieval and dark age soldiers. They always used their spears overhand (they state so explicitly during the middle ages, describing it as the 'ancient style' versus the way Normans couched their lances and such). It's a perfectly fine way to fight, especially in a tight formation. Underhand would be much easier to block. Overhand, you can manuever your thrust a bit better around defenses, at least to my experience.