Switches are normally closed as specified on the schematics, when used as limit switches one of them will open when activated , in that case noise won't be an issue. Used as Home switches and standing in a home position in a very electromagnetically "noisy" system it could be of some concern, in that case, a 1Kohm resistor from pins 1 and/or 4 of the opto-coupler to Auxiliary ground will help a little more.
Remember that home switches should be one for each Parallel port pin (unless your software moves only one axis at a time when going to the home position), you could add another opto-coupler IC and use other input pins for the other two axes. The diagram is only one example. You are not limited by the amount of series connected limit switches except for the fact that the more switches you use more cable is used between switches and more noise will be present in the circuit.
Other important consideration when chosing limit switches is switch hysteresis, mecanical switches have hysteresis, it shows as different home or limit positions depending on the direction of movement, (nothing to do with backlash), Opto-switches have the least amount of hysteresis and show no bouncing..