Shift Register Pin connections

I read through the examples regarding the 595 and 165 shift registers. There is some overlap of the pins used. Is there any particular reason why certain arduino pins were chosen to connect to the IC pins? If I just redefine them in the opening of the sketch so there is no overlap, will that cause problems?

In some cases, the SPI pins are used. They are assigned in AVR hardware. But if SPI isn't used, you can use any pins you like.