For bachelors and bachelorettes, is 8 hearts or 10 hearts considered "maxed-out friendship levels"?

8 hearts friendships with no bouquet will not decay. ConcernedApe's tweet confirming it.

You are correct, getting to 8 hearts with any marriage candidate will stop the friendship decaying. If you then gave them a bouquet, unlocking the next two hearts, it would start decaying again. Getting them to ten hearts would then stop it decaying once more, all of which can be done with every candidate prior to marrying one of them without any jeolousy penalties.

What is worth pointing out is that the decay is tiny. Talking to them each day gets you 20 friendship points, the decay is 2 points, so just talking to everyone at festivals would be enough to prevent the decay mattering, and actually increase freindship very slightly.

Ref http://stardewvalleywiki.com/Friendship