A few days before Apple released iOS 7, they start supporting download of older versions of your app. From the article:
For example, a user running iOS 5 can now — for the first time — download an app that supports that version of iOS, even if the latest version of the app is ready for iOS 7. The move, which was highlighted on Reddit, is a significant one as it will give those who have not upgraded their iPad, iPhone or iPod to the most recent versions of iOS access to a lot more apps.
So if I were you, I would do one more update to support older iOS devices for one last time before moving on and setting the deployment target to higher iOS version.
The weird thing is, I don't think we can go back and update the older version of our app. So if we have App 1.0 that supports down to iOS 5, then App 1.1 supports only iOS 7 and above, there is no way to release a 1.0.1 update to fix critical bugs for iOS 5 users.
That's why I said I'd update it one last time before bumping the deployment target. Maybe wait a few weeks and see if your last update has any critical bugs.