6.3V caps on 5V rail - not recommended?

[espresso]

Would it be safe to put 6.3V capacitors on 5V rails in PC power supplies?
I read somewhere that in PC supplies one should use 10V caps on 5V rails.
And for example, National Semiconductor's datasheet for switching regulators states:

The capacitor’s voltage rating should be at least 1.5 times greater than the output voltage.

Are there any voltage spikes that can harm the capacitors in the long run?
I bought some FM Panasonics to replace the existing caps but they are 6.3V rated unlike the original ones which are 10V.
Also, Panasonics have much lower esr than the caps I used before so I hope replacing won't cause instability.