I would think that you could take 20 on open lock check. The skill description says nothing about there being negative repercussions from a failed check. Sure, one could argue that you could break your picks or jam the lock, but those things usually happen due to inexperience on the part of the picker or attempting to pick the lock too quickly. If you're taking your time, you should have no problem. Of course, usually you won't have time to take 20 on an open lock check, but that doesn't mean you shouldn't be allowed to. (As I recall, Neverwinter Nights allowed you to take 20 on an open lock check -- although NN played fast and loose with a lot of mechanics.)