I appreciate a lot of different things in people, one of them is - Reliability.
I feel we need to stand by our promise/commitment. We are as good as our word. And just in case if we are not able to then, we need inform concerned parties about the change in plans beforehand. It is good to be considerate and also take accountability for our word. If you ask someone to meet with you at a certain time/day then it is a good habit to make a note of it in your calendar immediately, we can not tell them last minute 'oh you are waiting for me? but I left for shopping'.
I feel it is okay to say NO. Also for the receiving party it is easier to hear 'no' than to go through the disappointment and frustration that we will bestow upon them by saying 'yes' and then not showing up/ or taking care of something that we said yes for.