You can move corkboard tiles around to rearrange faces in whatever order you like: just click and drag them. But take care: if you drag one tile atop another, iPhoto merges those two faces: the face you dragged takes on the name of the face you dragged it to. (And there’s a sentence I never thought I’d type.) Fortunately, iPhoto can warn you about this. (As a safety measure, don’t check the Don’t Ask Again in that warning.)
If you do accidentally drag one face tile atop another, sprint right up to the Edit menu and choose Undo Merge Faces.