Calling someone out in a raid is a very tricky thing to do and I won't say you should never do it. However, you have to use it as a leading technique not as an anger reflex.
What is considered calling someone out?
The action of "calling someone out" is when you are vocally critical of a person's actions accompanied with using their name.
When should I call someone out?
When a player does something intentionally stupid, it is absolutely important that you call them out. This is not so much for their benefit as it is for the raid's benefit. When it comes down to it the type of person that does intentionally stupid stuff just shouldn't be in your raid. Calling the person out discourages this behaviour in other raiders.
When shouldn't I call someone out?
When a player does something accidentally stupid, you should not call them out. This will only shake their confidence and the confidence of your raid. Effective raiders need to be aggressive, proactive and responsive to change. This is impossible if they are worried about making mistakes.
Yup. Singling people out in a raid is something you should rarely have to do. 99% of issues should be directed at the raid as a whole not at an individual.