It's a matter of style, and depends very much on what you're writing, as well as on your personal preference.
In something more formal (most non-recreational non-fiction writing, and anything where you're communicating with someone in a formal context), I'd use the longer form, while in less formal writing (most often fiction or recreational non-fiction) many people would use the contraction. It really doesn't matter much, though; you're unlikely to get your head bitten off for using contractions even in something like a scientific journal article, and many people (myself included) prefer to write with the more formal long form most of the time, even when it's not strictly necessary.
That said, the one time this is important is when it is the voice of a character within your writing, in which the approach to contractions can be important in establishing the character. Most comfortable speakers use contractions; people new to a language (especially those with a formal education in it rather than an immersive one) tend not to. Other people who may eschew contractions are older characters and more "formal" ones regardless of age.
Bear in mind that this also applies when using a characterised narrator; either an actual character in the story (as with first-person narration) or simply an abstract character who is telling the story (A Series of Unfortunate Events has a good one).
Even people who normally use contractions can use the longer form on occasion as a form of emphasis; I do this a lot.
