Each state or Province will have their own regulation on lights, just like OSHA or the Canada Labour Code.
Rule of thumb: headlights and a back up light are mandatory when used after dark or in dimly lit areas.
As far as signal lights or brake lights: these become mandatory when using on a public road or street (I would consider these if the lift truck is taken off your private property).
Having said that: I like to see headlights left on all the time - day time running lights. Pedestrians say they can see the reflection of lights in an overhead mirror quicker than they can see the reflection of a lift truck.
As for the back up light: I think an employer should have a back up light that comes on automatically when the truck is put in reverse. Drivers say their biggest complaint is pedestrians walking behind their truck, "They know I'm going to back up, but they walk behind my truck anyway".
We need to get back to pedestrians and cars: if a back up light comes on, we tend to get out of the way. A back up light is sending "another" signal to the pedestrian, aid the employer and bottom line, we don't want to see anyone getting hurt by a lift truck.