There are 2 ways to create the chip select signals.
#1: Direct pin manipulation. All existing code uses this approach. Any digital pin can be used. It does not matter if one of the CS0, CS1, CS2, etc. The code merely controls a digital pin the normal way, so any pin be used. On Teensy 2.0 and normal Arduino boards, this is the only way. All the code which exists today was designed for those board, so none of it uses the other way.
#2: Automatic chip select control by the SPI port. Today, I am not aware of any library or example using this way. Eventually, some libraries may begin using this approach. When they do, one of the 5 special pins must be used. Each is a different signal, so in theory up to 5 different SPI devices can be connected this way.