On one hand, I'd recommend attending camps and seminars as a way of experiencing other instruction and other styles, rather than the "wondering ronin" approach... .
I skimmed though the thread so I apologize for any redundancy. With that said, from my point of view is the above is a reasonable approach. Because, I have found some senseis feel it is a matter of respect to tell them. For example, your sensei might have a rival and would feel it disrespectful for you to go. There maybe bad blood between the two, not checking with your sensei could mean trouble, as Senseis are human too. Therefore, going to camps and seminars can be a way to avoid that. Especially well known and high ranked senseis with good reputations.
One caveat is camps and seminars can be expensive and it isn't like paying a mat fee, at the most ,when visiting another dojo. Doing so, can be pricy, is usually over a weekend, and a bit of travel. That can be limiting to the number of times you can go, and which places you go. But, than again, if you go to someone like Joe Thambu sensei it is worth it. It is Aikidoka such as he who really give you a good picture.
Personally I look at a sensei in the same terms as a coach, mixed with se a private lessons teacher. There is a certain level of respect and courtesy afforded to their position (in terms of being out in other dojos) If they say "no" or get upset, that doesn't mean you have to agree, or comply. But you know where the sensei stands. You just don't want to unknowingly upset an apple cart. And, if the sensei is unreasonable about it, then it is time to change dojos. I agree it is always good to check with your sensei.