If you saw a credit card swiper sitting out in the open, you probably wouldn't be crazy enough to just slide your card through it and "see what happens" on your bill. Right?
By the same token, you shouldn't trust free charging stations -- there may be a device on the other end of the cable (e.g. hidden inside the desk/wall) that, once your phone is connected, can siphon off all your information and even upload a virus to your phone. In fact, over 360 people once fell for it at a convention.
If you absolutely must use an unverified charger, power off the device completely first.