One mistake I frequently see new forum owners make is to start like this: They sign up for their forum, start a "Welcome!" discussion, and then start a "Rules for this forum" discussion which they make an announcement and close. They've instantly crippled their site.
Remember grade school? I think everyone, at some point, had one of those teachers who had a big sign at the back of the room that outlined a half dozen "Classroom Rules" that stated painfully obvious things like "Raise your hand to speak" that anyone above the first grade knew. It does little but insult the students.
Here's the facts: The people who will follow those rules already know them and are insulted and bored. (Like their lives don't have enough rules to remember already - they're gone). The ones you're targeting will simply ignore it.
The caveat is with communities on a large scale. When you have a very large site, it becomes exponentially more challenging to communicate expectations to new members. The gold standard in large-community rules was set by Flickr with their community guidelines. It both addresses common, real concerns, and it sets the tone for the community. "Don't be Creepy: You know that guy. Don't be that guy." They've crafted a valuable resource rather than a boring introduction to their site. The guidelines weren't the first thing on their homepage on day 1. Deciding when (or if) to transition to written rules is an important decision.
Posting rules is the "easy out" attempt for setting the tone of your site. The bad news is, it's a lot more work than that and will take months or years of hands-on attention. The good news is, the reward is a truly great community, and that's priceless.