Using the contraction is a matter of the author's style and the type of writing. It's use is acceptable to reproduce the compressed speech of conversation or first-person narration. From Misadventure: Monologues and Short Pieces by David Marguiles:
Prolly not. (Pause.) Threw me outta school 'cause of it. Least if
I'd've been nineteen, least I'd've had a diploma from high school,
least I'd've got a real job, maybe. They dint wanna give what you call
special attention. Nowadays it's different.
I'd'vesounds almost exactly like
I'd havein spoken English unless you're a classically trained speaker with perfect enunciation and/or put emphasis on the space between
have. I imagine you'd raise more eyebrows by consistently pronouncing
I'd havecorrectly with a notable space in between.