@Charlaquin Just to be clear, the OP did not mention skill checks, they specifically stated ability check.
I mean, I still call for skill "checks" just because it's easy and I'm lazy. For that matter I still occasionally ask for a Reflex save.
So to answer the OP, I allow any proficiency and bonus the player thinks is justified or if I remember something such as their background or experiences. I admit though, I'm not 100% consistent as I'll frequently just give advantage on a check because that way it stacks with proficiency bonus if they have one.
This is more of a philosophical question than a direct question. Let's say a player wants to do something and there is no skill directly related to it, should you just go straight with it being just an ability check. or should you look for the closest skill and just use that. (Not the closest skill the player has, but the closest skill in the game.) The question I am asking is every action covered under the skill system, or not? How far should a skill be pushed to cover something not explicitly stated by it?
I mean, I still call for skill "checks" just because it's easy and I'm lazy. For that matter I still occasionally ask for a Reflex save.
So to answer the OP, I allow any proficiency and bonus the player thinks is justified or if I remember something such as their background or experiences. I admit though, I'm not 100% consistent as I'll frequently just give advantage on a check because that way it stacks with proficiency bonus if they have one.