In general I would share, though you can share with as small a group of people you trust as you want to get feedback, increasing in publicity as you're comfortable.
About people stealing your ideas — here's something that my mom told me when I was young and drawing a lot of cartoons, basically it goes like this: If you're a truly creative person, you never need to worry about somebody stealing your ideas. The only people who steal ideas are people who cant come up with them on their own. Those people latch onto ideas and never let go because it's all they have, when the creative person can come up with another new idea, and another, and another…
And you'll see people with ideas. Some have an overflowing basket full of ideas, other holding ideas people have a death-grip on just one idea because that's all they have.
So far in my schooling and career I've seen people that fit into both categories, so another 'layer' of protection is showing your ideas to people you know are creative. They don't need to steal your idea, and if you show them one of yours they might respond by showing you 1 or more of their ideas, or maybe showing your idea starts a brainstorm and you both think of new ideas. The people you would have to watch out for stealing your ideas are non-creative people who don't have an idea :D