Agree with others. I'm seeing a lot of people on italki use these terms and think its awful. I would personally never use any of these in writing or in speech.

Never in writing.

No you should not use them in a formal piece of writing. These are slang words that would only be used within a very informal setting with an American audience. British speakers would not use them and may object. I agree with Nanren that the user may be viewed as lacking education.

Never in a formal situation. Almost never informally as well. Anyone writing this way would be considered uneducated, as in almost no schooling at all.

Formal: Never For me? Never. . Informal ? Sure. If you want to be very informal and possibly be taken as someone without education. Depends on the audience.