We ordered ours online so we could do the Fast Pass+ in advance of the trip. We ordered annual passes though and are DVC emmebers, so I was fairly confident we would be able to have access to it (I don't think it's available to everyone yet).
Otherwise, I wouldn't get the tickets in advance unless some sort of discount is involved because then you don't have to worry about losing them while traveling and it's easy enough to get them at the hotel if you're staying on property.