Safari is / has been like IE in that it gets updated with the OS. They may release security fixes whenever, but new feature wise, it's updated with the OS.
All the sites I run / control - usage statistics are similar to older versions of IE - lower then Chrome / Firefox / IE > 9 - as such, it gets tested like IE < 9 - make sure it works if it can, but I'm not going out of my way to make sure it's perfect.
Safari for iOS - different issue - that gets tested as normal - mobile is important now a days.