My AES plug is rated at 100V, and the Japan plug on my converter is set to provide 100V. If you live in the US, the difference is 20V (US=120V).
It's not enough to fry your Neo, but if you're going to put thousands of dollars into your Neo collection, you might as well pay the $18+shipping to get the voltage exactly right.
However, I think pretty much all electrical devices are built to have a certain tolerance for having either too much or too little voltage. I don't think a normal household outlet consistently provides exactly 120V (or whatever) anyway, which is what I think the tolerances are made for. I just personally don't want to push my way 20V towards that upper limit constantly while using my system.
I'd worry about the voltage difference more with a CD system though, or anything else that contains moving parts.
Barring that though, I'd recommend just unplugging your Neo whenever you're not using it. (Good advice whether or not you're using a voltage converter.)