I don't think we had any issues at the Real Sociedad game, there was a mixture of supporters and no one complained about standing up. Like Stretty I think stewards were told to let is stand.That doesn't address the issue of what happens when one grumpy git complains to a steward that he can't see because the guy in front is standing, though does it? Either the steward has to tell people to sit down, or he ignores the complaint and the old git escalates things himself.
I think they'd take option A there, given the position they take in other circumstances.