Both. Before the game, if I want to change something about the system or the setting, I don't ask permission, but just about everything is negotiable if a player thinks it's going to make the game better for everyone. If I want to change something while that game is in progress, well, I'm going to try to make sure everyone's on board with it, especially if it's going to affect the way someone's been playing their character.
I love letting players expand their own options, as long as it doesn't detract from the theme of the game or anyone else's participation in it. For instance, I'll sit down and write you a whole 1-36 class (or racial class) if you want to add something new to my Spelljammer game, but I can be downright bitchy about trying to add stuff to Dragonlance or Dark Sun.