I think GMAT is important for scholarship or for your career goal in MC or IB after schools. Some schools will grant scholarship for merit-based such as Simon or Smith. Also recruiters will ask for GMAT score.
For School admission, I think GMAT is not the most important part. If you just graduate 2-3 years without stellar working experience or leadership exposure, 780 may help. If you have quality experience or different career track, a lower GAMT does not hurt. ( I know 690 gets in Wharton this year)
In my opinion: Essay and Interview are much more important than GMAT.