My own opinion is that an overhand (thumb-backwards) grip would have been relatively unusual and that for most purposes the control of the underhand would have been more important; a high underhand grip with the spear rested between the shields makes most sense to me as the "tight" formation, the overhand seems too unwieldy and the armour design seems to very much discourage overhand fighting (as it exposes the armpit, for example, which I consider a very major point against overhand being usual). The issue with the idea that overhand would be used when the formation was mobile is just that it'd then be quite hard to switch quickly into the shieldwall; seems more likely that the formation would move with it underhand then switch into a wall and slowly start pushing when they got close.
Of course there would be variation though, probably quite a lot of it. And you'll get people like Wolfy whose preference is for overhand because their muscle distribution favours it.
That said I should note that I consider the above a personal opinion not really a historian's opinion; if I get a chance to read some actual papers on the subject I'll get back to you on the latter
