As with any social settings I think the rules are pretty clear. Sure there are those who think that a simply No is rude however it is not. This problem arises from individuals insisting on perceiving reality as binary (black and white). No thanks, or no time right now are much more polite than a simple No absolutely but No should be a perfectly acceptable answer that is deemed neither rude nor polite a third category if you will. As a communication methodology the simple one word answer does have drawbacks and it is worth considering. To all those acting as though other people owe you help. Seriously? No one owes you help. Strangers are not required to help you move your couch. Being members of a community does not mean that other people are effectively your slaves.