I generally prefer to follow [Github Objective-C Conventions](https://github.com/github/objective-c-conventions):

> Always declare memory-management semantics even on readonly properties.

So I even made this explicitness everywhere by [this commit](https://github.com/stanislaw/kingpin/commit/0c4e724fd2f0c7ec2f77d53ff6f0e4cb1ecbc4cf). Let's keep it explicit, if you don't mind.