The cost of writing 'Hello' in an email is very minimal, the cost of reading one of these greeting is also very minimal.

The cost of being perceived as unfriendly, too-business-like, and demanding is just so high. It's not worth the risk.

PS: I don't care either way, but I have heard multiple people comment on how they don't like doing business with person xyz because their emails are extremely cold and unfriendly. Because of this I have alway add a nice greeting and a friendly closing. The middle is all business/meat, but padding it with some friendly talk can really help.