I always reverse the shortcuts to that copy is:
command + shift + (3/4) and save file is command + shift + control + (3/4)
Some other os x screenshot tips:
- hold space while dragging a selection and you can reposition the selection's origin
- press space to switch to window capture mode. This captures the whole window even if it is obscured.
Fun fact up until recently (mountain lion I believe) the capture window icon was a holdover from NeXT. I wish they'd left it alone, the new one isn't nearly as cool :)