Capacitor voltage in a 555 timer circuit
No, unless the leakage of the capacitor was affecting the timing, and in that case it would tend to make the time shorter. Or in the case of a high capacitance (Class 2 or Class 3) MLCC (multilayer ceramic) capacitor (see below). 

Assuming you're using an electrolytic (for example, aluminum, polymer or tantalum) capacitor, capacitance does not change significantly with operating voltage. Film caps too, but they are seldom rated as low as 10V. Also NP0 ceramic capacitors. All have quite stable capacitance regardless of applied voltage relative to rated voltage.  

The voltage number is a maximum rating, ideally a 10V capacitor at 9V acts the same as a 100V rated capacitor at 9V.

Here is an overview of capacitor technologies (see page 2). Note that none of them, with the exception of some MLCC ceramic capacitors, have a significant voltage coefficient. 

There is an exception in that high capacitance MLCC ceramic capacitors CAN have a significant voltage coefficient. One should never use a Y5V or Z5U capacitor for timing, but even relatively stable X7R and X5R capacitors can have significant voltage coefficients. The less stable types also change significantly with temperature and with aging. 

In the case of a ceramic capacitor of Class 2 or Class 3 (X7R, Y5V, etc.), using a higher voltage rated capacitor would tend to make the time longer, because the average capacitance would be higher at the operating voltage. 

(from the reference above)