Using Dialect and Colloquialism in Writing

They are appropriate during story telling.

Just keep your audience in mind. If what you're writing is fairly informal, such as a blog or a lifestyle piece, then using slang or regional dialects may be fine. Avoid using any words that might be offensive or unclear to those who come from outside your region, though. Certain phrases might need an explanation.

For writing such as school work, business or non-fiction I'd say keep it formal. It's better to err on the side of caution than appear unprofessional.

colloquialisms

The word describes the informal use of a language. It is not local or regional in nature, as dialect is.
Webster's New World Dictionary identifies many words as colloquial with the label Colloq. The label itself, the dictionary says, "does not indicate substandard or illiterate usage."

Many colloquial words and phrases characteristic of informal writing and conversation are acceptable in some contexts but out of place in others. Examples include bum and phone.

Other colloquial words normally should be avoided because they are substandard. Webster's New World notes, for example, that ain't is colloquial and not automatically illiterate or sub-standard usage. But it also notes that ain't is "a dialectal or substandard contraction." Thus it should not be used in news stories unless needed to illustrate substandard speech in writing. See the dialect and word selection entries. Good luck.