Sometimes, Evi will cry if I take her to the potty. My potty book, "Diaper Free Baby," says to respect if the baby tells you "no" when you put her on the potty, but I have found that not all cries are "no's." I just have to evaluate the situation to decide if I keep her on the potty or not. Sometimes she is crying because of the discomfort of pooping, and when I continue to hold her on the potty, she will eventually get it out and then light up the room with a huge grin. Those cries have more of a complaining tone to them.
If her cry on the potty really is communicating anguish, I will take her off the potty, even if I am convinced she needs to go. I suppose it is far better for her to use her diaper backup than to create negative associations with her potty. This happens mostly during the nighttime - the hardest time, I find, for practicing EC.