Voice - A Question of When

I usually write first person and my narrators tend to talk like regular people. Which means sometimes they say "kinda" and "gonna" instead of the more proper "kind of" and "going to".

In my mind, when I write first person, everything in the book is being "said" by my character, whether it's dialogue or internal thought/narration. Which means that everything should be said the way they speak. But I know others feel differently. Some writers/readers feel that anything that's not actual dialogue should be written in "proper" English. I can sort of see where they're coming from and can see times where this idea of separating narration from dialogue works better than others.

For example, a character who always says "ain't", and is surrounded by people who also, always say "ain't", would sound weird to me if when narrating or describing something, they used "isn't" instead.

So I'm asking for your thoughts on this. Which do you personally prefer? Keeping your characters narration in line with their dialogue, or separating them? Why?