I recommend changing the key to Super. As holding down Alt and clicking/dragging is often used by many applications and simply won't work then.
Yes, it is best to use Meta/Windows key for system related actions (copy/paste, screenshot, application start, various windowing actions), and let Ctrl and Alt be used by the applications.
I just learned that you can use Super + Left Mouse to drag windows around and Super + Right Mouse to resize, due to this discussion. I have been using XFCE forever, mostly using hotkeys for tilling, and just did not know :D
Thanks !