Actually what I've been told is a bit different. According to my conversation with adcom, the essay is an opportunity for applicants to express themselves and articulate their goals, but at the same time essays are an excercise in being able to follow directions. If they say 150 words then they want no more than 150 words. One example they gave is the 12-pt font requirement. They don't want to see 11.75-pt font or 11.5-pt font, because they asked for 12-pt font. Why would you do otherwise?
While some latitude on the (very low) word limit would be great, personally I'd do everything I can to stay within the guidelines provided. Just my two cents...