I try to judge it on a case-by-case basis. My kids are all different people, and each situation is a different one.
For example, when I'm changing a diaper and am up to my elbows in baby poop and discover that I need about four more wipes than I brought with me, and DD1 is sitting and watching me, and I say to DD1, "please go get me four more wet wipes," and she tells me, "no," my response would be, "get up off your duff and go get me those wipes right now, child." If she persisted in saying no, she'd probably wind up being threatened with some pretty sharp consequences that I would definitely follow through on, and also be subject to a pretty strong lecture on mutual responsibility in a family to boot.
But if I'm cooking and need a jar of tomatoes, and DD1 is busily painting, and I say to her, "will you please go in the cellar and bring up a jar of tomatoes," and she says, "no, I'm busy painting," my response would be, "okay, I'll get it myself, that's fine," because clearly my situation is not an urgent one. I don't NEED the help right now. I can turn down the heat under the sauce, and go get the tomatoes myself, and DD1 is clearly engaged in something absorbing and positive.
So there's no easy answer. I do think a child should have the right to say no, but I don't think a child, any more than anyone else, has the right to consistently refuse to accept the mutual responsibilities and courtesies of family life. I like the idea of letting kids try out "no," but I think that part of trying out, "no," is finding out the hard way that sometimes refusing legitimate requests gets you in trouble, or gets people angry at you, or makes people less willing to help you. They need to know that, too.
I do, also, think that when "no" is an unacceptable answer to you, that ASKING is dishonest and deceptive. It might not sound so courteous to say, "I need you to go get it for me now," but it's a heck of a lot more honest than couching your request in the form of a polite question, "would you please go get it for me?" and then getting angry when you're met with refusal. So if I really mean that something is non-negotiable, I try to say so--- if "because I said so" really is the truth, then I speak the truth. We've never claimed to be a consensual household, though. That's something I have no illusions about.