Ask Admissionado

Look, with a 3.9 GPA, 770 and being at McKinsey you are gonna get into businessschool, as long as your application and recs are solid. Yes, at least another year of experience would be sooooo good for soooo many reasons. You'll be a lot wiser going in, and a lot wiser going out. And probably you'll get a whole lot more out of your MBA experience. It might also give you enough time to secure you a return offer from McKinsey, which could be a sweet bonus as well, and a nice attractive little tidbit for the Adcom. something to look at when you apply is the average work experience in months.
Harvard:
Mean months of work experience of newest entering class: 47
Meaning if you apply now you are gonna be 30 months under the average!

So, I'd say wait it out at least a year.