Figuring out how early to reserve popular intercity trains during a busy period

We'll be traveling between major Japanese cities during a busy holiday stretch. I usually prefer buying train tickets on the day so our schedule stays flexible, but I'm uneasy about arriving at the station with luggage and finding the convenient departures full. We're not committed to a particular seat, though sitting together would be nice. Which journeys would you reserve as soon as sales open, and where is same-day flexibility usually worth keeping? I'll check the relevant operator's booking rules before committing.