Ahh, the jitters...
From personal observation, I feel the stress when I'm flying a really nice, (aka, expensive), plane and when I fly one of my beaters, (a Zagi for instance), I don't feel jittery at all. In fact, the planes that I don't worry too much about wrecking are the ones I have the most fun with.
I try and take one of each to the field so that I can fly the nice one and then take a break with the relaxing one. When I first learned, I can remember coming home from the field and actually feel soreness in the shoulders and neck! Talk about stressing out. But like it was mentioned above, that little nervous feeling will keep you on your toes and hopefully out of trouble.

Jesse