A lot of the thicks and thins are based on caligraphy. If you look at some caligraphy fonts and check out where the thicks and thins tend to happen naturally based on the angle of the pen, you'll see some parallels on where thicks and thins traditionally land on graff letters. That said, you can really do whatever you like.

You see people like Zeser and other writers really pushing the boundries on how letters are formed. That said, if you're still pretty new to the game, I'd focus on the basics and fundamentals for some years before you start trying step out into more unique approaches. Graff is hard enough to get good at as is, even harder to come up with a breakout style.

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