Do you allow comments to appear as soon as they are posted ? Or take a precaution and keep it until it is approved ?
I see in popular blogs that they cannot afford to put it in moderation. Very serious discussions start in comments as soon the post gets published.
I think a lot of this depends on three factors
The same as you said. When you can afford it, put it on moderation. But when you gets too busy or have published a catchy post, it is better to have the moderation turned off. Simply because you have no choice.
At the very basic level, there should be controls in place to remove profanity, salacious content, anything that could cause offence, or harm to another individual - such as defamatory content, or other types that can identify people personally.
Bullying is a classic example. You have to remember that comments can also be indexed, which doesn't bode very well for your reputation either - even if you never posted the content.