Work - should we learn to say no?

These are some questions that has been on my mind for awhile...

a) Is it good that we assist our colleagues on work that is not ours - when directed to assist as they can't seem to finish it?

b) Is it good that we are not able to say NO when 'asked' to do something because someone whose work it is in the first place can't do it?

c) Is it good that we complete work that is not ours, as they can't seem to do it?

d) Is it good that we are always available to the office?

And lastly,

e) Is it good that one's own superior does not appreciate nor seem interested in what is being accomplished or NOT in the office?