In my experience nokia has quite a big head room on input voltage, i have charged my N900 & N95, with an input of just 4Volts, thats in the USB on the N900 and normal charging plug on the N95, this was just a test a few months ago, and it is not the method i use to charge, now on the "overvoltage" side, i would think you can go up to 6Volts or more, not like my old SE phones, they want between 4.6 - 5.4V else they give an error, meaning that they don't break or brick.

But if your voltage is off, and this is on the USB port side not the Mains then the N900 should give you a warning, and remember almost all chargers for many things these days accept between 80-250V (mains input) so even with power swings the N900 charger should still keep a fairly stable 5V output.