I have turned on account moderation. I actually turned it on a few weeks ago, but then turned it off during #railsconf to make it easier for attendees to join.
However, less than a day later, an account appeared posting links which clearly broke the instance rules, so it's back to moderation again.
It's not actually that much trouble to approve the accounts, and it's vastly preferable to having to suspend abusive ones, so overall I think this was the right choice.
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