I always found the credit score needed to get the best rates subjective because the banks decide what they consider prime. I personally think anything above 720 is good enough but 760 covers your bases; again, my opinion as a disclaimer. As for a buffer I don't like getting into that mindset because then you try to optimize your scores whether that's AZEO or what have you.
If it helps you keep to a schedule as far as paying things off and wanting to see score movement then go for it. I do not begrudge people finding ways to stay on the path to keep their scores high.
As stated in the previous reply, it's all lender-specific as far as their cutoff score for the best product at the best rate(s). Whatever that number is, 720, 740, 760 etc. I would say it would be wise to have a score of 25-30 points higher than that in order to be able to absorb the addition of a new account from time to time and still be able to achieve that best product under the best terms.