Should I run a line from each injection point back to the power supply, or can I run one line to the start of the prop and branch out to my injection points from there?
A lot depends on the total power being drawn and the size of the cables.
I run only 4 wires from my power supply to my mega tree injection points and split off at the bottom of each string, but the cables I run are heavy enough to carry the full load for the total of what is being drawn.
So as an example, I have 4 strings of 75 pixels 5v, theoretically I would draw 4.5 amps per string or 18 amps total on full white.
So I would use 20 amp wire and split it off at each string.
Obviously this is more helpful on yard props such as my spiral trees, where they are only 20 pixels high, so I inject the whole row of them with 1 main wire.