I know Japanese males tend to use 俺｛おれ｝/僕｛ぼく｝ in informal contexts. Is it common to use 私｛わたし｝ too or it makes one sounds overly stiff and aloof?
Edit: What about 自分｛じぶん｝?
I think Japanese males don't use watashi in casual conversations but females use it.
If males use it in casual conversations, it may make one sounds overly stiff and aloof as you say.
自分 is often used in Japanese sports community. They has strict age hierarchy in regards to using polite manners, it is called 体育会系. So 自分 includes the meaning of humility. It is rarely used in formal but there are some people using in casual conversations. They are probably 体育会系.
In addition, 自分 also means the second person(that is to say, "you") in Osaka.
In informal contexts it is normal for men to say "ore" (俺) or "boku" (僕). "Ore" is more "manly" and more likely to be used by older males.