You want your dialog to sound convincing. Think about how people actually talk. One thing most of us use on a regular basis is contractions. Not using them can make it sound stilted. For example, would you say “I am going to the store. Would you like to go along?” or “I’m going to the store. Want to go along?”
Saying it aloud helps. We don’t always pronounce all the letters in a word, either. Back to the previous example, how many would actually say, “I’m goin’ to the store. Wanna go along?” Using an apostrophe to indicate missing letters shows it’s not a misspelling.
If one or more characters have an accent, be sure to reflect that phonetically as well, even if your spell-checker gags a bit. This is another way to differentiate between speakers, if your characters have distinctive speech styles.