Maybe this can help?
Most of the chests are Lockpick level 3 I think. In the beginning I only put 1 point in lockpick in Farglow to open the chest nearby the Mage and I put a second point around level 8 or 10 (or later). I put a third point in Lockpick just before I start taking over the Battle Tower, because of 1 chest in Broken Valley Village is L4 (why I don't know). Since I know Jagon's Necklage give 1 point in Lockpick I hold that Necklage as long as possible.
I have make a list of all chests, but not their Lockpick-level . I count around 150 chests (and 15 small box'es ) for Broken Valley & Sentinel Island you can find before taking over the Battle Tower.
You can find many "keys" to open locked chests, so I always first search everywhere so I never missing a key.
I do like you: try first to kill some enemies. But since I love the "trading" aspect in RPG's and for sure in the DKS(!) I always try to do first the Mindreadings and/or Quests for a trader so he or she lowered his/her prizes ! and many times you can solve a problem (quesst) but NOT turning in the reward for that quest ! So I do to have "Locke" as a nice cheap trader very early.
But I never took the extra XP when going for a Quest Reward. I always took the extra Gold
Good Luck & much Fun with your nightmare mode playthrough Hassat Hunter !
