Backwards LEDs don't work
Can you tell where the cathode goes on this pcb image? The answer depends on whether the diode is a rectifier, an LED, a uni-directional TVS, part of a daisy-chain and a host of other considerations.
Have you ever had an LED or other diode placed backwards? By us? By someone else? By yourself? We don't like it when that happens and we're sure you don't either.
To ensure the best accuracy, we recommend extra care in marking your diodes to remove any ambiguity. Simply placing a "C" for cathode adjacent to the cathode on the board works well, as long as the diodes are clearly designated as diodes (D1, D2, etc.). You can also use an "A" for anode or the complete diode symbol in silk screen.
Relying on +, - or _ are not definitive in what they indicate and are not recommended.
Don't smoke your LEDs