Well, I think I've seen images of vases showing other ways of holding the spears as well, it isn't all using the same method.
Mostly likely, all the hoplites in a phalanx (or from a given city, say) used the same type, but reality is messy and it wouldn't be surprising if different cities might have had different forms of spear use that are roughly similar and all worked well enough.
Of course we'd probably just want to pick one animation and use it for all hoplites