I've never had a problem running pedals from the USA with 'normal' pedal power supplied.
As long as the voltage, current and polarity are correct for the pedal, I don't think it matters as far as I know.
I can understand it for amps; USA amp runs at 120v, UK amp runs at 220/240v. I don't think it matters so much with something as low as 9v - 15v