If there is a seat, I am sitting. That's the purpose of said seat. If someone stands in front of me and I'm not able to see the show without standing, then I'll stand without complaint. Otherwise, if there is a chair available for me to sit in and my view is not obstructed, then sitting is what I shall do.
I've never been an advocate of the rule that standing/sitting is dependent upon the specific genre of music that's playing. That being said, I'm also of the opinion that you should expect the liberty of freewill. There's no law that says that if there is a chair, you must sit. So, if someone wishes to stand - regardless if that person is immediately in front of you and your preference is to sit, then that's their prerogative. If I have to stand because the person in front of me is a stander, then I'll do so and know that I'm not going to die in the process.
To me, it's just a matter of going with the unpredictable flow of a concert setting. And you know what? If it's a show like the 4.5 hour marathon at the Beacon, where the boys played all of SFAM and all of Master of Puppets and then came back out to play Metropolis, and I'm losing feeling in my legs, I'll sit for a few minutes and then get back up if my view is that severely obstructed.
No harm, no foul, to me.