Depends really;
street and dirt jumps; narrower bars for sure which is definitely a trend borrowed from the BMX boys, nothin like the super easy x-ups, fakie to half cabs and control when manualling you get from narrower bars.
DH; wider bars are the go, until the point you find yourself struggling to get the bike between trees, or poles at the end of sets of stairs
Personally I run about a 25.5" effective width, this is for an urban / DJ / DH hardtail and I find it to be a good compromise between the three disciplines. I'd love to go about an inch narrower, but it would make the bike a lot less useful for downhill.