My 2 cents worth:
"NO" is a power word, as is "YES". In order to maintain their power they must be used properly - and sparingly.
If you say "NO", and then qualify it with conditions (either verbal or otherwise) that might indicate that it can become "YES", you have diminished the power of the word.
Likewise, if you say "YES", and then not follow through, your use of that word loses it's force and meaning. You are no longer a "man/woman of your word".
I try to never use these words with my boys unless I am absolutely sure that they indicate what I mean. Sometimes I think that they have heard "perhaps" or "I don't think so" so often that they want to scream when I respond that way. But I want them to know exactly where they stand when Dad says "NO"; and I want them to know that if I say "YES" they can take it to the bank.