Opinions vary, and like most things in life, change with time.
I was always taught to signal as I passed the exit before the straight-on exit which I was taking, and was also taught to signal right if going right round, switching to left as above when passing the exit before the one I was taking.
Some people are taught only to signal for the exit they are taking, which in my view is incorrect, because there is no way to distinguish you from the hundreds of numpties who don't signal at all.