Are seat reservations recommended?
On many trains reserved seats do not get booked out, but on some they do. On particularly busy travel days (e.g. peak travel days during Golden Week, Obon and the New Year holidays), trains can get booked out several days in advance, but on most other days trains rarely get booked out more than a few hours in advance, if at all.
For peace of mind seat reservations are always recommended, especially when traveling in groups and preferring to sit together. On a few shinkansen trains along the Tohoku Shinkansen (Hayabusa, Hayate and Komachi) seat reservations are mandatory.