Hi,

Not shielding is like kicking a sleeping dog, at some point it's likely to wake up and bite you.! For the sake of a little wire, it's not worth not doing it.

The main areas to use are Stepper cables and limit/home cables or any cables that carry low voltage signal cables.

Here are a few tips to help you avoid EMI :

#1 Star grounding is a MUST DO, this means all Gnd's and shields, etc go to a single Earth point.
#2 Keep low voltage signal cables, like step/Dir, limits, etc away from Mains voltage wires.
#3 Use shielded cable and only ground one end of the cable at the control box end.
#4 Earth the frame.
#5 Use 24vdc for signals like E-stop, limits, homes, etc.

Now you say you are using a booster of the PC PSU, this is a bad idea and could cause problems because you are tieing the 24Vdc Gnd to the PC Gnd which can cause ground loops. For the sake of £10 do yourself a favor and buy a dedicated 24vdc PSU, you only need one 3a at max for the low voltage side.