I haven't had that problem, but I suppose it's the same worry that ragtop owners have with people cutting their cover... My wife had a Miata, and never had a problem, though sometimes I was worried when parking in bad areas... On the other hand, it wouldn't take somebody much to bust my canopy windows to steal stuff in my bed. What can you do? Buy what you like, and use your street smarts. The rest is just trying to be prepared for a zombie invasion that may or may not happen.
Unless someone has knowledge of you having valuables in the bed, I don't see them risk getting busted just to cut it open when there's no guarantee there's anything WORTH taking. I bet you'd be more likely to have your window smashed, at least then they see in it first if you have anything worth breaking in over.