Can I shout excelsior! when the in memory of stan lee credit pops up in the next marvel movie?
If you want to but why would you do that when that happens?
Because Stan Lee is known to say that phrase.
You shouldn't shout in the movie theaters because it's rude. Even though it's the end credits people won't appreciate a lame joke.
I've got the right to free speech and I'll do what I want with it.
Even though it can be rude to others, I do see it as harmless, so I guess the end credits are the most appropriate time to do that. Don't scream really loudly because you don't want to spook or scare people. You are courageous for being willing to do that.