Usb chargers will be rated at 5v, which is exactly the correct voltage to charge your device. What you want to make sure is you buy the correct "rated" amperage. Not because it will damage your phone. Amperage only exists as a sum of the power used by the device. You want to find a decent 1amp (1000mA) or higher if you wish, rated usb charger. The the mains charger for your phone is only rated at 1amp, so a 1amp charger is adequate. Avoid 500ma chargers as it will take twice as long to charge and gps / satnav applications will drain the battery even when charging at that rate.
Also if you have a new pc, most of the decent boards will specify 1amp charge even when off if it has on/off charge stated on manufacture details.