Question: Is there any reason why or why not you should do an 'order by' in a subquery?

Answer: 
You should not use it. According to the 'Art of SQL', this actually prevents the optimizer from performing various optimizations that it could otherwise do because this pre-maturely transforms the data.
It's totally legit. I.e. `SELECT id FROM entries WHERE author_id IN (SELECT id FROM authors ORDER BY name DESC)` but you'll really get the same results usually.