Is it better to write a 650 word essay than, say, a 600 word one? Obviously, you want to write something that’s quality over quantity, but if you had a 600 word piece, should you look to stretch it a little and make the limit?
No. Quality>quantity applies here as well.
No, you should not. Write so that you have a complete piece that you are proud of. If you ramble to fill space, 1, it is obvious and 2, it will lessen the impact of what you you really want to get across.
These adcoms read for a living. They recognize a “stretch” from a mile away.
Write the essay you want to write, not the one that fills up the 650 word limit.
Adding 50 words will not increase your chances of admission. Simply use the number of words necessary for you to best convey your ideas.