5V signals can be susceptible to noise as Jonathan says, however provided you're sensible and don't do stupid things like run the signal cables next to power cables, most people don't have any issues.

If you're using Mach, and looking for lots of inputs for non-critical stuff i.e. buttons to make things happen where a slight delay isn't likely to cause disaster, I'd look at getting a PoKeys - POKEYS IO DEVICES
Things like limit and home switches are critical, and have to be directly connected to the controller to get reliable operation.