Before I went, I checked the yen exchange rates at different places everyday in my home city. There was one small place related to a travel center which consistently had the highest rate and no commission. So I figured I'd just get the majority of my cash exchanged here, and then if I needed more than I could exchange in Japan.
I did that, and didn't regret it at all. When we got to Narita Airport, we found the rate was slightly worse there, so we were glad we got it exchanged back home.