Actually hoods for wide angle lenses tend to be petal-shaped, as that's a better design for them (and it's not needed with normal/telephoto lenses, where the AoV is narrow enough that a round hood does the job). But since hoods for zooms must be optimized for the wide end (except in the case of the reverse zoom found on the original 24-70mm), because they'd vignette at the wide end if they worked well on the tele end, those hoods tend to be petal-shaped. Even the 24-70 hood has a petal shape for the wide end, it's just much deeper because of the reverse zoom (meaning it works well at all focal lengths, unlike the hoods for all other extending zooms).
Technically (IIRC, epsiloneri pointed this out in a previous thread), a petal design would be optically best for any focal length lens, but practically speaking for normal and longer lenses, round is just as good (because of the AoV is small enough relative to the front element size), and allows the lens to be set hood-down more safely.
