My editor at St. Martin's is an Executive Editor, and she seems to get what she wants. I talked to someone who used to work there, and was told that Senior Editors and Executive editors (whatever a particular publisher calls them) still has to answer to the publisher, but when it comes to picking graphic designers and publicists, they tend to get their way. I'm still in the middle of the whole process, so I have no idea how true that is, but I did like the preliminary cover design, and I've noticed that when I ask for things that I think seem to be pushing my luck (such as adding a photograph to the book itself), I've been surprised at how quickly I've been told yes.
But I've also been told that yes, sales and marketing wield a lot of power.