It all depends on the weight/expense/rarity of the object you are stealing. If they are meaningless items such as ammo or caps, you don't need to have a sneak very high. If you try to steal a weapon, on the other hand, you will get caught more often than not.
What if it's a lot of caps. Like, you know... 18,000?
I got Old Lady Gibson to repair all my stuff, but couldn't steal my caps back, so I cut her head off with an axe. Then I had my caps, and Fully repaired gear. Huzzah!