I don't think should really plays into the equation. Like PP said, as long as they follow the rules, they shouldn't necessarily have to post a certain way. That said, I, for example, am almost definitely not going to be pulling the same kinda bitchy arguments or just general rudeness as I might've used to—but, like Roundy, this is totally voluntary and self-imposed and has nothing to do with what I think a mod "should" be like. It's more about holding myself to a higher standard than thinking it's what I'm "supposed" to do.
That said, I think the most important thing is that mods still be casual and friendly. There is nothing worse than losing members to moderation, where suddenly they don't post as much and make themselves scarce and seem afraid to post regularly as they normally would. Being a mod doesn't make you special. The worst kinds of mods are the ones that get all huffy and develop egos because they have power and get to decide what's "right" and blahblah. That's my least favourite kind of mod.