If you go parallel, you have to watch out for overloading the LEDs with current. For a small number of LEDs (like less than 20), series is fine, and it keeps the wiring really simple. In my experience, when an LED fails, it often fails closed, so it stops lighting, but still passes current. If an LED does fail open, it would be easy to find it by just using a couple batteries and wires to check each LED. It wouldn't take more than a couple minutes to figure out which one(s) are bad. I wouldn't worry about that.