Generally, when somebody says "Hi", my reaction is "Oh dear, here comes the dumb question" and I cringe.
9/10 Times I'm right.
Generally I tend to /join and then just sit there for a while to discover the norms and protocols of the channel before speaking, unless I have a specific question/problem.
If I have a specific question/problem then "Just ask" is the best approach, but sadly, 9/10 times I end up fixing the problem myself, and get roped in to help solve 10 other peoples problems while I'm there!.
There's a rare occasion when you'll /join a channel and they'll greet you, but thats part of the 'norms and protocols of the channel' thing, and its usually a hallmark of a small channel, because having 500 users who come and go, if every one said 'hi' , or we greeted each and every one, $topic would never get discussed, and we'd be a 24/7 meet-and-greet channel!
This is especially good when there's only a few people talking, as fewer people => more interesting conversations (usually).