Apple made developer accounts free (at least to some degree), so we can actually compile from source now through Xcode 7.
I wouldn’t fully trust that, no, but I do (or will) anyway.
I don’t use Circle, but say I actually began paying with coins, I would choose to trust some wallet in the App Store. Compiling oneself and updating just isn’t practical. Wouldn’t keep all my funds in it, though.
I encourage paranoia, but in practice I think it’s not going to be a huge concern for most people. I do wish it were, so there’d be a more forceful push toward practices of such transparency. Say verifying that an app is built on the code we can view. Something like Gitian, maybe.