More is definitely better, but the "virtual dice" the game rolls to determine success seem to work against you a lot more often than you'd expect. There are a lot of modifiers that affect a steal check, notably the size of the item, whether or not the person is facing you, and (though I could be mistaken) the target's perception.
I seem to recall Fallout's engine imposing a hard maximum of 95% on success chance for skill usage, as well, which means that you're always going to have a 1/20 chance of failure for all those other modifiers to amplify.
There's a perk (whose name escapes me at the mo) that eliminates size and facing penalties from the equation, but with the irrational frequency at which Stealing seems to fail, most people will tell you to save a perk slot and a bunch of skill points by getting Steal up to somewhere between 60 and 80 and just save scumming with the quicksave slot if you're considering a career in the larcenous arts.
(Sorry to keep jumping in and blathering out book-length answers to these. Insomniac jag.)