I would keep that trimmer head flat. You're most important job is to keep the lines exactly even, cutting them doesn't work. Are you using a bounce head? Or a straight head? I prefer straight heads without shields...when you are more proficient.
Before using your trimmer for the grass, turn it on, turn the entire thing upside down, using concrete or a boulder, touch the two lines to the hard surface very lightly. Watch your downstream...always.
You will hear the engine RPM rise and steady and then those lines are perfectly sized. If you don't do this you will be harming your motor eventually.
They should be no more than 4" long each! Then watch the true cutting power of a line trimmer.
Always, always no exceptions wear safety glasses and ear plugs. Always be aware of your surroundings and where you are throwing grass and stones on your downside. Always.
Use non ethanol gasoline. Use a good 2 cycle oil that is measured correctly each and every time. Too much is better than too little. Oil the bearings in the head. Make nice with the repair guys! They can teach you an awful lot as well as put you closer to the head of the line...of clients. I use star orange line...see how that improves the performance.
Tilting the spinning lines on the side gives you a swath that will never be even. As you cut the grass with the trimmer pretend you are a robot. Fix your arms at the height you want to cut the grass and don't scalp that grass! Always aim for 3 to 3/12" high, if you are cutting cool season grasses?? Use your gas powered blower to blow off the clippings onto the grass then mow over them to suck them up and bag them. Always bag clippings.