Well it wasn't a groan of displeasure, it was more along the lines of "damn, was the totem going to fall or not?" A gasp of frustration is probably a better way of describing it. I thought it was funny.
As for my take on the ending, I agree with your wife; the totem definitely seemed to be starting to wobble before the cut to block (hence the frustration). However, a lot of people are pointing out that it was Mal's totem anyway, not Cobb's. Some people who think that the ending was reality have suggested that his kids were his totem. In the dream he never sees their faces, whereas at the end they turn and run to him.