I'm not sure that C-z is a great choice as it's often used for job control. I use C-o myself. I suppose you could always use C-z C-z if you wanted to suspend your current foreground process.

as an emacs junkie I use backtick for my screen control. Works fine till you try to paste blocks of SQL containing loads of said backticks - easy way to fubar a screen session.

Perhaps its emacs' fault but finding an easy to use key or key combo that isn't already in use is far too hard!

I use ctrl-_ (aka ctrl-7).