Using Japanese video game system in US?

I just got back from Japan and picked up a Famicom (Japanese version of Nintendo) and PC Engine Duo-R (Japanese version of Turbo Grafx 16/ Turbo Duo). Both have all the original cords and AC adapters, but is it safe to use these in the US since the voltage is 10 more volts here(110 US, 100 Japan)? All I could find online is that it "should" be okay because the voltage isn't that much of a difference. I'll admit that after reading that I couldn't help myself and tried both of them out and they worked fine....but I am still a little nervous that the higher voltage could eventually blow them up or something. I thought that the actual system was only pulling 9 volts of electricity anyway since it says its a "9 volt adapter" on the plug itself. Anyone else know for sure if it is safe to use these or not?

I'm pretty sure it's ok, I use my Japanese PS2 without any special stuff (other than the cords it came with) and I haven't had any issues.

A 10 volt difference is barely more than the duracell battery you used in your walkie talkies as a child.