I just returned from Japan and had -zero- problems with my chargers. All but one stated 100-240 on the wall-wart, but even the one that isnt multi-voltage (120v only) had no problems at 100v. 100-120 is so close, that I really dont think it makes much difference. I never had an issue with the ground prong, as all my adapters are two prong anyway. That being said I saw grounded outlets (and three prong adapters) everywhere.

My Japanese friend has many American appliances, and uses them all in Japan. He also did the same in the US with his Japanese stuff when he lived here for a few years with no issues.

The only potential problem is if you are bringing with you a motorized appliance (fan, blow dryer, bench grinder , etc) as the difference in frequency (50hz vs 60hz) will make it run slower and a little hotter.