From playing originals in a plethora of noisy toilet circuit rock bands, I'd say that the performance is as much about the energy you create on stage as anything else. They may be tired old rock poses, but in the context of a balls-out rock band, it's much more appropriate than standing still.
Most people who go to gigs aren't musicians, and they won't define their memory of it on you missing a few bass notes, or leaving the hard bit out and playing root notes so you can machine gun the audience with your headstock. They'll remember a band that not only play great songs, but perform with energy and engage the audience.
What this means for you depends on the band you're in, I guess. You'd be a bit out of place playing behind your head whilst doing the splits in a trad jazz band, but sitting on a stool looking at the fretboard won't do much for a room full of rock fans. So long as you enjoy it, that's the important bit!!
EDIT: Wotnwhy just said the same thing as me, but far more coherently, whilst I was typing this nonsense. Well played sir!