iOS 7 hits 85% adoption20
That metric has just been updated, and as of now, 85% of devices that checked in to the App Store have been updated to iOS 7. iOS 6 has dropped down to 12%, and the final 3% are on iOS 5 or earlier. It seems as though the iOS update numbers have essentially plateaued, and the changes that we're seeing now are simply due to users trading in old hardware for newer devices. Back in January, the number stood at 80%, so there hasn't been all that much of a shift since then.
So, developers still likely want to build their apps for iOS 6 compatibility, but depending on what kind of app you have, it may be perfectly reasonable to build specifically for iOS 7.