Reserving seats with JR Pass

Hello,

Quick questions for those that have (maybe) experienced this ... Is it possible, once we land in Narita and got our JR Pass to make reservations (with seats) for at least 5 of our trips using trains in Japan? We will arrive there for a 3 week holiday and would like to be able to book seats for the trains we will be taking in the last week of our trip as it is bang on the golden week.

I am afraid the desk will be quite busy with so many people landing at Narita and getting their JR Pass and doing the same. Is it a better idea to wait we get to our first station outside of Tokyo and do it there or there is no point as if most people are doing that then we should just go ahead and do it too!

Thanks for your help,

Yona