I suppose it depends on the stature and submission guidelines of the magazine. The New Yorker and Asimov's for instance, wants the whole story (for fiction anyway) and I really doubt they'd steal it. It's good to be cautious, but magazine's don't stay in business if they're ripping people off.
Have you ever checked out Predators and Editors? I always check with them before I send anything out. They're great!