Eye to that, that you put it in more places than you think, in the DIC, in the Kernel, etc.
Moreover, I recommend you see this excellent talk by Marc Morera:
Likewise, some best practices have been published:
Notice that it talks about getting into the parameters.yml INFRASTRUCTURE parameters:
Likewise, it tells you to set parameters in the config.yml that will be able to change with the enviroment (Environment), mainly.
And it talks to you about constants vs. configuration parameters.
And lighter, water:
Use constants to define configuration options that rarely change.
# app/config/config.yml parameters:
const NUM_ITEMS = 10;
Personally, one of two; or would keep it in a class constant, or create a property of the class (private), and set it in the constructor of it, which can be redefined in the constructor of it (thus, even , you could redefine it in the services.yml of the Bundle, if you are going to define it as a service -eye to this, always use the definition as Proxy; Ocramious has a great library for this link -); and, of course, I would not use absolute routes, unless it was strictly necessary.
I hope it helps you.