The "Electrical constraint set>static phase tolerance" in the constraint manager is what controls this. Click on the button that says"ns" and select "mils". Then type in the tolerance you would like. I usually type in 10 mils. Remember that "10 mils" will give you +/- 10 mils window. So any given diff pair may have 20 mil difference between the two trace lengths. The smaller the tolerance you give the harder it is to hit that "window" when you are sliding the traces around. You can also use delay tune in the single trace mode to "stretch out" the shorter trace so it is a closer match to the longer trace.
Hope this helps