No, more is not better. The voltage rating on a capacitor simply indicates the maximum amount of voltage that the cap can handle. Guitars have very low voltage circuitry. In theory, a basic rule of thumb is that every capacitor with a voltage rating of say 1 volt or higher will work in a passive electric guitar. Usually as the voltage rating increases, so does the physical size of the capacitor. One reason for the higher voltage tone caps that you find in some guitars is because a lot of popular caps like the Sprague Orange Drops were used in tube amps with higher voltage requirements; such as 600v or more. These caps sound great in a guitar, but they are huge. An Orange Drop cap with a 10v volt rating would sound just as great. So, whether it is a 50 volt cap, a 100 volt cap, or a 600 volt cap, it would sound the same.