Ceramic Capacitors vs DC Bias - Derating rule of thumb misleading?
There are many reasons for selecting a capacitor's value, voltage, size, and technology.  A design starts with understanding the requirements which include voltage, power, frequency, size limitations, temperature range, cost, etc.  Then what are the limitations of the capacitor (or any component) which are found in the specification.   Why does capacitance decrease? It is an effect called Voltage Coefficient of Capacitance and the effective capacitance decreases as voltage across the capacitor increases. MLCCs are constructed using alternating layers of ceramic dielectric and metal electrodes.  Class II MLCCs are made from BaTiO3, a ferroelectric material, which has a crystalline structure containing Barium, Titanium, and Oxygen atoms.  At low voltage the Ti ions are free to move according to the electric field.  As voltage increases the ions begin to line up and become less free to move reducing the effective capacitance.  (Film caps BTW do not exhibit this behavior.)
So you need to consider your highest working voltage, both DC and AC.  Second consider the upper temperature range and the effect on leakage and how much leakage your circuit can stand. After determining the required voltage I may add another 20% margin.  Recently my company has undergone a major investigation into cracking of ceramic capacitors.  We found that smaller capacitors are less likely to crack, but as you showed, smaller capacitors have higher variation with voltage.  So you have to decide. 
For those designing a home project, 50% works.  For everyone else, read the data sheet.