This is my experience.
Pumping while doing passive things, like reading or watching tv, not trying to get/stay erect in the pump, this feels the least pleasurable, and I don't feel the stretch as much. BUT that may not even matter, since feelings aren't how you gauge how good something is working. When I pump this way, I tend to see more of a girth increase, but that may be because I'm not tempted to go into higher pressures, and I can stay at a low pressure for a long period of time, which from what I read, gives the best girth gains.
If I pump while stimulating (either having something external get me aroused while pumping, or playing with the pump a bit), so that I get erect inside the pump, it feels the best and I feel a good stretch. The gains seem slightly limited when I do this, though.
If I pump after ejaculation, I feel the most stretching occuring. Almost as if My penis is resisting the pump. it also feels the least pleasurable, almost painful at times (mostly due to the heightened sensitivity that I get after ejaculating, it's even hard for me to enjoy regular stimulation after ejaculation unless I have some down time). The thing is, when I pump after ejaculating, my penis remains in a larger state for a longer period of time.
Everyone is different, though. Whatever works best for you, works best. Just don't go on feeling alone (unless you feel pain, which always means stop). It might feel great, but that doesn't mean you are getting a better stretch.