I just finished watching 4-6 then 1-3. ROTS would have been entirely ruined if a new viewer had watched it before the OT. When Vader's mask is put on and R2-D2 and C-3PO are given to Captain Antilles, it wouldn't have meant as much nor made sense to someone who started with the prequels.
Emotionally and logically, it all works a lot better going from the OT to the PT.