Simply put, do not use a quotation to replace your own words or ideas. Avoid profanity in a formal essay. Many people praise author Richard Rodriguez's novels, but I think he's a fucking sellout. This sentence fails to critique Rodriguez's work and makes a personal attack at his character. A more effective statement is, Richard Rodriguez's novels embrace assimilation and perpetuate racist ideologies.
Profanity is acceptable in creative pieces or when quoting someone else.
Yes, however, try to avoid it. The reader dismisses the statement. It's not a fact. Thus, your statement does not convince the reader.