When to purchase Main cabin extra for international flight?

Newbie here. I just booked my first international long haul flight (from US). It is a basic economy so I can’t upgrade, but I’m able to select a main cabin extra seat for a fee. The prices have gone down since yesterday, so I’m wondering if I should purchase now or wait to see if they go down more? Or even wait till last minute and see if we get assigned those seats for free?

Secondary question, are they worth it? Right now they start at $78.

Here are my stats:

Plane - Boeing 777-200ER Height - 5ft even (husband is 5’5”) Destination - From Charlotte to London Flight time - 11:25 PM October 12, 2023

Any insight or advice would be greatly appreciated!