Personally, I would advise paying for it. The matter is important enough for you to go to the trouble of starting a thread about it here. By paying for it upfront you can relax and know it is 'in the bag' and you will enjoy peace of mind until the day of the flight, in addition to your chosen seat assignment.
And if it turns out you discover you could have waited until T-7 and saved your money? No worries, you still enjoyed peace of mind up until then. Let it go!
Hope you have a great trip whatever you decide ^