If I got a 36 composite score on my ACT but a writing score of 8, should I retake the ACT in hopes of a higher writing score, or is this good enough?
If you took the ACT September 2016 or later and got an 8/12, then no, you should not retake the ACT for a higher Writing score. A 36 is a phenomenal ACT score, and schools care about your composite score far, far more than they do about your Writing score. And while low Writing scores with high composite scores can be a warning sign to colleges that you struggle with timed writing assignments, an 8/12 is high enough that this shouldn't be an issue.
The only case in which you might want to retake the ACT for a higher Writing score is if you got an 8/36 on the essay (if you took the ACT Sept 2015-June 2016), since that is relatively low and might raise concerns about your writing ability. However, since there were so many problems with ACT Writing scores last year, even then I would urge you to spend more time perfecting your admissions essays rather than retaking the ACT for a higher Writing score.