Nope. Yelling isn't okay. Parents are human and may do it anyway, but it's always a mistake and should be treated as such (except in the case of danger as mentioned above). It absolutely should be avoided as much as possible. It's an incredibly harmful thing to do (for any relationship, actually).

Child walking onto the street in front of a truck? Billy please come here, please don't walk into a truck. Never is such a strong word. And I have a mother who yelled, frequently, and I still think there are a few cases. Parents aren't infallible or perfect.