Hello or NoHello? It's a thing. Some people want the social lubricant, some don't. The first thing to remember is that Slack (or any other asynchronous communications method) is not a face to face, real-time, discussion, so the rules/norms/best practices are different. It's also much more likely to be remote/cross time zone :)
The two biggest things to remember in chat are
1) It's very hard to communicate emotion/expression/intent, which means the failure mode of "clever" is "jerk"
2) Asking if you can interrupt is already an interruption.
John Scalzi handles the first one well enough that I'll leave that to him.
The second is more community driven and we collectively need to figure out what our norm is. Like anything else group based it's a consensus/compromise. It's also situational. Expectations are different in a 1:1 discussion about a new work issue or about last weekend's new movie, or a request in a large group. My position is nohello, or if you feel it's rude/wrong, then preface your question with the salutation of your choice, then immediately ask the question in the same message. Notice that I don't say hello before I send out a post here. So what do we want our norm to be? Share your thoughts in the comments.