To answer the question - when to use the rocket launcher.
I'm fairly new to the game, so my simple answer is - always use it when your opponent's is close to a wall. The rocket explodes on the wall (obviously) and the splash dammage gets him. When playing against single player bots running to pick up health, knowing this allows you to kill them every time.
Another point - when up against a lightning gun or sniper, small, random straffing works well for me. You don't need to jump,
The rocket launcher is far less easy to master than it first seems, and judging which piece of ground is is fairly difficult. Master the flak cannon first, then move on to the rocket launcher.