Look at most of the monsters.
+1 and +2 DR are the most common types. +3 is more rare, and usually found only on powerful outsiders and dragons. +4 is very rare, and I think is on powerful celestials. +5 is on the Tarrasque and Stoneskin.
Since most of the monsters have +1 or +2, sure striking will be most worth it in extreme cases. Your base +1 weapon cuts through alot of DR, and if you enchanted it to +2, then you'd not only have an additional bonus to attack and damage, but you'd also be able negate most DR.
Making Sure Striking a +2 property means that, instead of a +3 sword, they could have a +1 surestriker. The sure striking weapon is at -2 to attack and damage, less hardness, etc. In exchange, the weapon can kill celestials, Tarrasques, and guys with Stoneskin easier. Beating stoneskin isn't worth a -2 to hit and damage.