How to use your Q spell:

For wave clear you can use your Rocket Launcher for the back line minions before your waves pushes the meeles, because your minions can't deny you farm then. On long range pokes you of course use your Rocket Launcher and for fight you mostly stay on your minigun if it's single target. If it's a team fight with many people on the same spot, you use your Rocket Launcher and you melt them if you have Hurricane aswell. For general farming and saving mana costs you of course use your Minigun.