[i]I believe[/i] that when you're playing in front of an audience, you owe it to them to put on a show, or at the very least look like you're enjoying the gig even if you really aren't, after all, it's showbiz isn't it? Even if it's only down the local pub, it's still a live gig, and people have taken time out of thier day to come and watch you.
I've always hated bands that stand motionless staring at thier feet looking miserable, I really hate that.
So my vote is for showing off, every time.