How Far in Advance for Shinkansen Tickets?

In general, how far in advance should I book Shinkansen tickets if I plan on reserving a seat, especially if I'm planning to take luggage with me onboard?

Would it be okay to do it the morning of? The day before? The week before?

I'm interested in people's experiences.

I don't have experience regarding the luggage, but in general it strongly depends on the time you are going.

During golden week and new year you should book well in advance. During normal weekdays seats should be available up to the trains departure. If you are traveling in large groups, this might not be the case however.

To sum it up: Do it as early as possible :)

Have your RP exchanged as early as possible (starting dates can be in the future) and make the reservations right at that point. I always have the connections I want to take printed out, so this really takes just some minutes.
Some reservations can be made online, the Railpass-Page on this site is excellent and will tell you everything you need to know about this.

You can always cancel the reservations and book new ones.
But if you can forsee that you will not use a reservation please (!) cancel it before the trains departure as your seat will not be sold again.

And my thoughts on the luggage restrictions: Sum of all sides can be 160cm. Our suitcases are quite large, and they do not reach these figures. In fact, I recently wanted to buy new suitcases and hardly found anything that was nearly 80cm high. So, most probably you won't need to have a prebooking for your luggage (that that is just my thoughts, I have not traveled with my suitcases after 2020).

Awesome answer! Thank you very much!

I plan on taking the Shinkansen on a Monday and Thursday in mid-January 2023, but from what it sounds like, it's best to just book in advance and cancel it if I need to.

Thank you very much again!

1 month is the maximum. Just bought tickets Tokyo Kyoto for 31/Dec and wanted to buy the return ticket for 9/Jan at the same time but couldn�ft because sales weren�ft open yet. So bought the return today.