On the command line, I like to keep my DISPLAY variable set so that I can remotely run X apps on my workstation. There is one exception for me, though, and is emacs. With DISPLAY set, starting emacs will automatically start xemacs, along with a 2 or 3 second delay as the initial window is drawn from across the internet. To avoid this and force emacs to start in console mode, simply create an alias:
alias emacs="emacs -nw"
The '-nw' forces emacs to ignore your set DISPLAY.