If you're going to post in these communities, you'd do well to remember that they share many users. Therefore, if you make a post where you come off sounding like a complete jerk, people are going to call you on it. I've seen it happen in both communities. Heck, I had it happen to me once in customers_suck. I learned, deleted the post, and moved on.
It's one thing to vent about suckiness. A waitress getting pissed off because you ask for more water at a sit down diner is definitely sucky. A customer who demands a sale price on an item that isn't on sale sucks just as much. Venting about these things is a-ok.
But words are the language here. You can't dehumanize people and expect them not to get angry. You can't use words that indicate hostility on your part during the interaction and then complain that the customer/service person reacted poorly to it.
Just some food for thought.