Definitely worth reserving. The Japanese meal is always the first option to run out on all these flights. I am flying LAX-NRT this week and do the flight very regularly. It is almost always better than the western options, but you can be pretty much guaranteed that if you end up wanting something other than the Japanese meal, you can easily switch because there will almost always be someone on the plane that wanted the Japanese meal but could not get it because they ran out.

One thing to note is that the Japanese meal is not really typical "Americanized Japanese food", but usually has several ingredients and preparations that are not commonly seen by most Americans that are used to the standard "teriyaki chicken and California roll" type of fare, so you may have things that are very new and taste different from what you are used to. Nothing really weird or gross, but just different. Tastes pretty good too.