Change the default application (for a file extension) via script/command line

To create an account in the Users folder

dscl / -create /Users/addfromcli

Create and set the shell property to bash.

dscl / -create /Users/addfromcli UserShell /bin/bash

Create and set the user’s full name. (Displayed in some of the panels/login screen)

dscl / -create /Users/addfromcli RealName "Added From Cli"

Create and set the user’s ID. (this is the unique id, you need to find out what id your last user create was and add one)

dscl / -create /Users/addfromcli UniqueID 503

Create and set the user’s group ID property. (This can be your default group)

dscl / -create /Users/addfromcli PrimaryGroupID 1000

Create and set the user home directory.

dscl / -create /Users/addfromcli NFSHomeDirectory /Local/Users/addfromcli

Set the password. (dont use password as a password)

dscl / -passwd /Users/addfromcli PASSWORD

Some more details can be found on OS X Daily

There's no real reason that I can think of not to leave it on all of the time, but you can easily turn it on and off by loading or unloading the launchd plist:

This will turn it on:

launchctl load ~/Library/LaunchAgents/com.ecamm.printopia.plist

This will turn it off:

launchctl unload ~/Library/LaunchAgents/com.ecamm.printopia.plist

Or you could just run the server directly:

/Library/PreferencePanes/Printopia.prefPane/Contents/MacOS/Printopia Server.app/Contents/MacOS/Printopia Server

(that's all one line)