I generally think that if the car is not " that" broken, don't fix it! A 690 is totally acceptable, and it seems like the rest of your professional and academic work would suggest that you can handle the quant at the schools to which you are applying.
When I was in admissions, I generally suggested that students let us know if the quant was unnaturally low for some reason than would not have been readily apparent,( illness, etc...) and in that case I would have also suggested a class in stats or finance to help to support the application.
In my opinion, optional essays can be used to explain some gap in work or school experience ( you were looking for work in a down economy, or you were sick while in school and needed to take time off) or to explain a sub-par undergraduate experience ( for a good reason) Its not a great place to talk about a 690 GMAT, which sort of makes you sound more apologetic than you need to be.
Thanks, and let us know if you have any other questions!