And you can see both in core :)

https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/Fixtures/yml/full.yml#L6

https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/Fixtures/yml/full.yml#L40

The same is true of `config.yml` in the standard edition. So, since it doesn't matter, let's just be as consistent as we can. Since True and Null are using single quotes, let's just keep this.