I rarely play anymore, just a heads up. With 3 jobs, college, and a girlfriend i'm swamped to be completely honest.
As for rocket technique and tips here's a few that I can say...
Rockets are splash damage weapons. You'll be aiming at the ground, next to a person 90% of the time.
1.) Never have 2 rockets on at the same time, it's a waste. Almost any smart player can avoid you especially in team matches, granted that's only 20% of the users on gunz. I'd suggest Shotgun/Rocket.
2.) Start off using only your shotgun at first to gauge a person's lead and to take note of their movement styles so you know where they're going to land and predict a person's next move, this is probably the main key to being able to use rockets is to predict what people are going to do and when.
3.) Practice with the rockets and get to know how close you can shoot them to yourself before you get hurt. Most people when they see a rocket user either A) Stay the hell away and play off the walls all the way across the map, generally people with tank sets do this. or B) Get close and try to mainly sword the person to take away the rocket completely.
When people try to butterfly one of the best steps you can use with a rocket is; look at which way the person is approaching you, and halfstep backwards/away from the person and launch a rocket towards where you are standing/in the direction of where the person is coming from. This will generally cause the most damage + it may also knock a person back and throw off the rhythm of a person.
When 2v1ing Someone: Don't chase into a hallway, stay in the open. I know it's easier to hit a rocket in a close corridor but it's also easier to take damage from your opponent that way also. Stay back and let your partner do the tanking. What you're going to want to do, is stay in the air and take as little damage from your opponent as possible. If they're a swording opponent more than a ranged opponent wait till they're relatively close to you're teammate and then launch a rocket at your teammate or halfway in between them, it'll cause splash damage which is all you'll be wanting to inflict at this time. I'd save about 2 or 3 shotgun shots in your gun to jump in between you're team mate and your opponent when your teammate needs to reload / heal. Chipping away at health is normally the best way to win in a 2v1.
When you're 1v1ing with rockets, what I normally do is use the angles to launch my rocket. I shoot rockets in a Z pattern. By this is mean, the two people are the end points of a Z. While you SS to the left, you will GENERALLY see your opponent SS'ing to the right, which means you want to launch a rocket while you're halfstepping to the left where they're going to land on the right.
I launch my rocket to hit in between the two points when i'm not sure of their playing style, in case of a halfstep so it will do some damage either way. But as i said before if you know how they fight you'll figure out if they're going to halfstep and land back where they were or just do a slashshot so you'll be able to predict their landing correctly.
When in a 2v1 against you: Keep moving. As all 2v1's you must keep moving. Most people will expect a rocket user to backpedal and run away while launching rockets behind them to hit. This is exactly what you must do, but you have to make it look like you're making an aggressive head on attack at them while doing it. You can back pedal a little, but you must keep going dead on at them, using the walls and corners of walls for your rockets to hit them. Tight ally's are the best place to 2v1 with rockets obviously, but also you take a lot of spam damage from their shotguns, so that's why I play in the air and use the corners of walls, to not get hit by their shotguns but also inflict damage on them while getting away.
More to come, tired.