Here is an interesting coin of Helena (Constantinople RIC.33) which shows the augusta clearly represented looking upward.
And if you take a look at RIC.33,34,48,49, you can see other specimens looking upward.
http://www.nummus-bible-database.com/rechercher-une-monnaie.htm?page=3&personnages=24&ateliers=9&collections=&vendeurs=&motscles=&numric=&numnbd=&legendes=41&nombreResultats=110&btRechercher=Rechercher
So, the question is : Is there any significance for this representation of Helena ?
I think that it was probably just the engraver's decision to portray the bust this way. I have seen many Constantinian coins with the "eyes to heavens" that I think were just artistic license by the engraver. Below is an example of a Constantinopolis. I have also seen VRBS ROMA with the head tilted.