When do y'all decide when to use a diode in a circuit ??

I'm wiring some e-fans for a bud, putting in an manual bypass switch. I'm wondering if I should diode that line, as I can see it feeding back thru the circuit.

Reason I ask is he got illuminated switches, and I'm betting they'll glow anytime the fan is on, regardless of it's position.

Unfortunately, I know just enough to always double-guess myself