Should PoE be turned off on ports not needing it?

If the PoE switch is 802.3af compliant (most are, but there are other implementations out there. I believe Cisco has their own) then it's safe to enable PoE on every port.

Basically, the PoE switch will check for a certain resistance between two of the pairs of wires on the other end of the device. If the resistance is within a certain tolerance, the device provides PoE power to the port. If the resistance is out of tolerance, then it does not.

Unless you have a very weird fault on the other end of the line, then the switch will not trigger its PoE functionality for that port.

PoE is negotiated between the devices on each end of the link, so there is no danger of the switch supplying power to a device that doesn't request it.

http://en.wikipedia.org/wiki/Power_over_Ethernet#Powering_devices