Turning off default new window on terminal startup
In Terminal's General preferences, go to profiles and create a new profile (let's call it "exit"), then under the Shell tab check the "Run command" option and enter "exit" with Run inside shell checked and "When the shell exits" set to "Close the window."



Then under General preferences, under "On startup" simply choose your new "exit" profile.  This will still briefly open a window, but there's no way to have Terminal on launch do absolutely nothing.

You might want to check out iTerm2.