ntzsch wrote:
when to write an optional essay=when you have something to say.
+1
An essay that comes out overly-contrived could hurt your application more than help. It's the same mindset that goes into avoiding mediocre LOR.
However, there are a few schools that it is fairly well known you should at least try to come up with an essay that sounds good (Michigan, Penn, UVA, for instance - I don't know your numbers or what schools you are specifically looking at). If a school asks for an optional essay, I would try to write one, then give it to a few people to read and see if they think it works.
If a school
doesn't ask for one but you have a particularly good diversity statement or "Why X?" essay to write then go for it. But it better be pretty good.