Despite all of the Saw movies containing multiple flashbacks and a few of them manipulating the audience's perception of time, the series should be watched in the order in which they were released. The main portions of all of the Saw movies are basically in chronological order. The only exceptions are Saw III and Saw IV which occur at the same time. Also, most of the traps in Jigsaw take place before Saw, but half of the story takes place after Saw 3D, so you might as well watch it in its release order. Another exception will probably be Saw X which is said to take place between Saw and Saw II.