I suppose I have a slightly different take on it. Cultural differences mean a lot here I think, and ways of saying hello, thanks, and so on vary a lot between them, and also between people in the same culture. I have a fairly high confidence that I don't know enough about anyone else's preferences to assume anything based on their behaviour, so much like Chew I just go for assuming good faith.
If someone says "hi" to me, I'll respond, because I think it's courteous. If they don't respond to me, I'll assume either they can't, or that they don't think it's a polite thing to do, or that they think it's hypocritical, or whatever they want to think really, it's not going to bug me. I would assume either they don't realise I would feel that way, or that they are on a client that means they can't respond. It's possible they are choosing to be deliberately rude / offensive, but I consider it unlikely, and even if they were it's their problem, not mine.
I tend to start my games with "Have a great game

" because a) I actually enjoy great games, and b) I just like being irrationally friendly to unknown people online - I think it sets a good first impression and a pleasant atmosphere, and that's something I like. The irony is, whilst I
do wish them a good game, I'd be kidding myself if I denied that I'm also selfishly doing it so it sets a tone I personally feel more comfortable in.
Live and let live I guess

Just my 2p / 2c