Derating tantalum capacitors is important when dealing with older technology. While a 16V electrolytic capacitor may have been sufficient for a 12V line (such as one of the hard drive power lines on an LC475), a 16V is not recommended by the manufacturer. They recommend derating the voltage and temperature (if possible). Voltage plays a much bigger factor in early failure than any other possible cause.
So how do we do it? Look at the graph below for how which voltage would be best for you. Typically 50% is a good value to follow. I’ve been looking at capacitors and picking their voltages from the right hand column and then selecting the voltage in the middle column. Works pretty well. Polymer tantalums aren’t really viable at this point due to their (even) higher cost than regular tantalums.
Check here for an explanation from one of the leading manufacturers of tantalum capacitors. Which is also where I grabbed the above screenshot from. Make note of the likelihood of failure after 60% of the rated voltage. The figures speak for themselves!