Were I to get an air, I feel the 64gb stock version would more than suffice for me. Taking into account room for office, some misc apps, I would save mostly documents (that don't take up too much room) or use external HDDs, or rely on dropbox. That would still leave my 40-50 gb of space to play around.
I don't bother with photos or videos, nor do I need space for bootcamp/parallels/games etc and once in a blue moon when I may need to edit a video, I would need 10gb of temporary space at most.
That said, it just costs me an extra $100 to get 128gb SSD, so I may be tempted to just spring for upgrade. 🙂