12V fan operation at 8V

[Martin Czech]

Sometimes it is usefull to cool circuitry with fans.
Most of them operate with 12V (at least the smaller ones),
most of them now have electronic steering (commutation sp?),
and I assume that pretty flat dc voltage is expected by
the fan designers, because they are used with switched power
supplys. 

Now the question:

Will it be harmfull for these guys to run at a lower
voltage, given that the rotor is still spinning?
Will large ripples <12V be harmless?

Will it limit lifetime?

I'd like to save a wire or two, and have nominal 8V...

m.c.