Which is interesting to me considering how untrustworthy he is.

OTOH, it's not like he's actually that good at lying. When he's talking his way into a crime scene or whatever, his believability is only so-so, and he's even worse when he's lying about something he cares about - to Sam, say. (Although he's still better at it than Spike.) When it comes to looking someone in the eye and telling them lies, Sam's much better at it.