If you manually pick a lock (safe applies the same, but lets just address locks to keep it simple), you gain skill.

If you force a lock, you gain just a tiny bit of skill, not nearly as much as doing it manually.

The harder the lock is, the more skill you gain.

The easier the lock is, the less skill you gain.

The % chance of success when auto-forcing the lock is based up on how high your skill is.  Don't think of each lock having its own %, think of it as a total.

Here is a visual (but not accurate) representation:

100 total skill - Can auto force any level 1 lock with 100% success

200 total skill - Can auto force any level 2 lock with 100% success

300 total skill - Can auto force any level 3 lock with 100% success

400 total skill - Can auto force any level 4 lock with 100% success

500 total skill - Can auto force any level 5 lock with 100% success

600 total skill - Can auto force any level 6 lock with 100% success

700 total skill - Can auto force any level 7 lock with 100% success

So lets say your skill is 343, that means that you can 100% a level 1, level 2, level 3 and have a 43% chance of nailing that level 4.

So, what does this all mean?  Skill level builds with each one you do manually, is it *that* important?  Nah.  It helps your successes for other petty/felony/OC crimes.

But we do so many of these, just do whats easiest.  If that means just auto forcing locks when you have a decent % and you are feeling lazy, then that sounds like a good plan.

OCs are based on the amount of petties/felonies you have done.  Since lockpicking/safecracking increases your crime skill when done manually, even more so at a high level, and even more so when done really well, it increases your crime skill, which is the main stat OCs focus on for success.