A switch can and many times is activated by axis movement in either direction! In my particular case, I have one switch on the X axis mounted in the middle of the saddle. Thus whenever either end is approached, it will trip the switch. I also did the same for the Y axis. However for me, it was easier on the Z to have a switch for each the Plus and Minus directions due to the way the machine is made.
As for homing, you tell your controller in which direction to move to home. The only time you will get into trouble with a system with switches on both ends is IF the switch is already tripped on the wrong end to begin with because the controller will think it has reached the home position and start to "back off" the switch but in acutallity, will drive further into that switch position. Having said that, I have not had that situation happen to me in the several years that I have had it setup. You just have to pay attention to what your doing.