I second the "slightly wider than the shoulders" sentiment. In fact, I'd go as wide as it comfortable, especially on
1. a SS bike bc you can gain leverage in climbs with slightly wider bars. at least i feel i do anyway.
2. an aggr AM or FR/DH bike for similar reasons to 1, but it also aids in control in sketchy sections at speed.
The downside of course is narrow trees but you can deal with that with a quick side to side move. Just time it right and make sure your pinky is not swingoing out there in the breeze 🙂
Anything less than shoulder width would feel totally awkward, except perhaps on a road bike... even then I'd still want it shoulder width.
p.s. I saw a couple of XC racers on Sunday with really, really narrow bars... besides looking odd, it looked as if it offered less control.