Agreed that it all depends on who the narrator's supposed to be, and, frankly, what tone you're trying to bring across to the reader.
Two of my favorite opening paragraphs of all time:
When age fell upon the world, and
wonder went out of the minds of men;
when grey cities reared to smoky skies
tall towers grim and ugly, in whose
shadow none might dream of the sun or
of Spring's flowering meads; when
learning stripped the Earth of her
mantle of beauty and poets sang no
more of twisted phantoms seen with
bleared and inward looking eyes; when
these things had come to pass, and
childish hopes had gone forever, there
was a man who traveled out of life on
a quest into spaces whither the
world's dreams had fled.
H.P. Lovecraft, "Azathoth"
Contrast with... this:
Tyler gets me a job as a waiter, after
that Tyler's pushing a gun in my mouth
and saying, the first step to eternal
life is you have to die. For a long
time, though, Tyler and I were best
friends. People are always asking, did
I know about Tyler Durden.
Chuck Palahniuk, Fight Club
Perhaps not a lot of slang in either, but talk about different tones. Lovecraft famously wrote of stuffy, deliberate intellect clashing with cosmic horror, and his narration is somewhere between sonnet and scientific-journal entry.
Palahniuk, meanwhile, reads like a wild-eyed young man, slightly out-of-breath, who's just flung an arm around your shoulder and started telling you a story.
Think about how you want your story to sound - like the confession of a learned man, or like the antics your buddy related to you at the bar, and let that guide the way your narrator speaks. And the emphasis on sound and speech brings us to a classic tip: read your narration out loud. Nothing else will make the awkward bits quite so obvious quite so quickly.