So you need to use thick enough material depending on how much strength you need for that particular flight. I started building model rockets when I was in the 6th grade. That was about 1976. There are so many variable that could affect the altitude of a rocket. Fin shape is one of the least ones.
Fins are most effective when they are in smooth flowing air.
This type of air doesn't allow the fins to be as effective as possible. The fin tips have better chance to be moving through smooth air. Therefore, that portion of the fin is more efficient at creating lift than the root of the fin where it attaches to the tube. Your eyeball is pretty good. Also, the human hand can feel subtle differences between fins.