I was just there last week (8/10-8/14) for my first time up Mt. Whitney. I didn't get the impression that theft was much of any concern there. Everyone I encountered there were just other hikers doing the same thing as me, and everyone seemed nice. Of course, anything is possible, but I wouldn't worry too much about people stealing things. Just use common sense and don't leave valuables laying out in plain sight - put them in the trunk or hide them underneath boring, uninteresting stuff in your car, like papers/mail, a cheap jacket, etc. You should be more concerned about curious bears than people. There are many bear-proof boxes around the parking lots, which you can use to stash any food and scented items (such as lip balm, hand lotion, etc) that you don't take with you on your hike, so that the local critters will leave your car alone.
Hope this helps. Good luck on your hike!
