Can I still use the obsolete APIs?

After updating my project from Unity4.7.2 to Unity4.5.2, many obsolete APIs were auto-fixed, but still some remain leading a lot of warnings.
Like this: Assets/Standard Assets/Water (Pro Only)/Sources/Scripts/Water.cs(91,28): warning CS0618: UnityEngine.GL.SetRevertBackfacing(bool)' is obsolete: Use invertCulling property’.
Must I fix them one by one? Can these obsolete APIs still work?

They will still work but on some point they will be removed. If you don’t want update you runity version you’ll be fine. if yes, your code will stop working.