NewDay's suggestion has always worked well for me. Probably because Motivation Units is about the only item I don't redeem right away. That might meet everyone's needs.
However, if the devs consider implementing this suggestion (putting a small new sign beside the new heroic deed reward) then it would be a good idea to treat it like a list of emails. By that I mean, go ahead and put the NEW sign as suggested, but also allow the user to MARK AS NEW after reading it. The user might not want to redeem the reward at that moment, but they might want it to remain highlighted so they can find it again when they do want to redeem it.
In other words, go all the way in treating it like a list of emails. It will make the feature even more useful, and users are already comfortable handling listed items that way.