Do I always have to discuss conference attendance with advisor? As a PhD student (in the US), should I always discuss whether I should go to a conference or summer school with my advisor before I apply? Even if 


The conference is providing full financial support, and
I am not currently employed as a research assistant (rather teaching assistant) and the conference wouldn't affect that. 


I realise it is common courtesy to do so anyway, but is it still OK if I don't?

Please don't answer to tell me what the "rules" are, or with "why don't you want to tell your advisor about it". I'm rather looking for a practical answer.
Practically, no, you don't always have to contact or discuss your advisor. It is possible that your advisor will not end up even knowing. Now, this isn't advisable just because you and your advisor should really be fostering an open and collaborative environment and your advisor might take this as a sign against this. Additionally, if you are submitting work, many would say that it is your advisor's business because they should be able to know most things about your research and career in order to properly advise you.