Booking in advance with rail pass

Hello,

I will be arriving in Tokyo on the Saturday and am still weighing up whether I'll get the most out of a 14 day pass. If I choose to get a 7 day pass and ask for it to be activated on the Wednesday, when I arrive on the Saturday, can I make reservations on the Shikansen at the same time or will I have to wait until the Wednesday before reserving any seats? As I plan on taking trips around commuter times, I'd rather reserve as early as possible.

Many thanks,

Kat