To reserve or not to reserve bullet train

I will travel with one person on the Yamagata shinkansen from Oishida Station to Tokyo on Dec 28. I would like to be flexible with the departure times, especially if I want to spend more time in Ginzan Onsen. Would it be easy to get two seats in the non reserved cars or should I reserve?

I would say around that time, you'd probably be safe in jumping on the non-reserved cars. I'm assuming you're using JR passes though? If there's still seats available in the reserved cars, you can just get reserved seats right when you get to the station to depart to Tokyo.

Use this if possible.
http://www.jreast.co.jp/e/ticket/reservation.html
Already reserved seat ticket sales started fr Today(JST GMT+9).
(just one month before)

Now checked JP booking condition web no sold out.
But Non-reserved cars very crawded usual.

Personally, I never reserve seats of bullet trains in normal occasions, but I would recommend you to reserve in this case, only for the specified date. At the end of the year, many people try to see their family and the bullet trains could be crowded.

Normally it is easy, but not confident on the 28th of Dec. My answer.

Thank you guys! Really helpful info here. I think I'll reserve just in case.