Do I always have to discuss conference attendance with advisor?
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.