I never use Quinn so my affection with him is embarrassingly low. So I was wondering if it makes more sense to leave it at that and raise it after a certain terrible action of his or max it before the encounter. Thanks!

Quinn is the #1 reason we should be able to kill our companions.

 

...

 

#2 is Kaliyo.

 

Really it doesn't matter. Affection gains are so incredibly large in value compared to the hits you take, really it boils down to how much use you want out of him for gathering/crafting on your way to endgame. Because even after the "incident," 1-2 decent gifts will have him capped out again.

I meant the story not the gameplay. Are there any extra dialogue if you raise his affection to 10k and did all his quests?

The only difference I can see in Quinn's dialogue options are whether you romanced him or not and how far along the romance arc you are. There may be a slight difference but it's not huge with the level of affection or how much of his conversations you've completed. There's a dialogue tree for the "incident" somewhere on the forums, Bright_Ephemera was kind enough to put it together, but I can't remember if it centres more on the romance or not.