Should I book my Shinkansen ticket in advance or buy it when I arrive at Shinagawa?

I’ll be arriving at Haneda Airport at 9 AM on October 24 and plan to take the Shinkansen to Osaka afterward. Since I’m not sure how long immigration, baggage claim, and the transfer from Haneda to Shinagawa will take, I’m wondering if it’s better to reserve the Shinkansen ticket in advance or just buy it once I get to the station.