Here is a great image that show exactly how to wire up one of

3D Printing
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1

Here is a great image that show exactly how to wire up one of those micro switches used for end stops. If you are struggling to find the proper way to wire them up this very simple diagram is the answer!
http://4.bp.blogspot.com/-qsGlYf11sfA/Urkpf76f6MI/AAAAAAAAALo/Bal6o2CQlJc/s1600/Screen+Shot+2013-12-24+at+12.08.32+pm.png

2

There are actually 2 “proper” ways to wire them up. One is with a constant 5v signal that goes to ground when the button is pressed, the other is a constant low signal that goes high when pressed. Both are definable in the Marlin firmware.

3

Defining proper is risky … Fail Safe would be better. E-Stop practice in industrial equipment will 'Fail Safe" if a circuit is broken to the switch/sensor. For example - an E-Stop switch will provide current to a relay coil so if the cable is cut or damaged the relay will de-energize causing an Emergency Stop condition. In the case of Z minimum having the head crash into the bed because the switch circuit was damaged would not happen.

4

Indeed, Normally Closed Switch with constant 5V ‘cut’ by the switch is much safer.