The official canon hood for the 18-55 isn't a petal hood for one very very important reason, the front of the lens rotates!
Not sure on the correlation between petal style and zooms, never heard that statement made before. The petal style is just a way of pushing the protection from stray light to the edges of your rectangular frame, rather than just the round shape.
#1 reason to buy a hood is to stop stray light entering the lens from funny angles, to maintain image quality. Buying a wrong sized hood will either not achieve this or will vignette which obviously does not maintain image quality.
So for your 18-55 don't buy a generic rubber hood, don't buy a petal hood. After that feel free to decide between canon and 3rd party