A quick look at one of several radiation/disaster effect maps of Japan shows that the key areas to worry about are all North/Northeast of Tokyo. A wedding South of Tokyo would likely source the majority of its foodstuffs and other supplies from Southern-side sources, and of course the stigma of using foods and components from the affected Fukushima region will likely ensure that it is the case.
Overall, visiting Japan is safe as ever, perhaps in some ways more expensive, and even now still a bit harder to get from point A to B than it used to be, depending on where you’re going. Outside Tokyo, especially to the South, things should be fairly “pre-tsunami” normal… But depending on the date of the wedding itself, the monsoon season, or the near-tropical summer clime might aggravate your experience a bit, or even cause direct safety issues (flooding, heat exhaustion, etc).
Long story short: if you can go, go. It’ll be safe, and you’ll be in one of the most amazing countries to visit as a tourist, for a once in a lifetime event with friends. What could be better?