OmniFocus looks for changes on the server every hour, but it also sends any local changes every minute. (And if it sees any changes on the server when it's sending local changes, it will pick integrate those at the same time.)
Although it would slow down the quit process, I do think it would be reasonable to have a sync-on-quit option for those who want it. If that's something you'd like, please vote for that feature request by sending email to
omnifocus@omnigroup.com. Thanks!