Presentation inc 'dressing for the stage' should be appropriate ( Seasick Steve appears in a manner that suits his minimal music style ). I played in a dance band (folk) that looked like they'd just fallen of the sofa! - I always wore a smart black shirt newly laundered, and I felt better for it.

As this is 'art' I expect the performer (artist) to dictate the gigs content ( requests within their known repertoire obviously included ). If they change their music this usually 'broadcast' by CD content.

I try to be 'professional' in my approach from song choice to delivery, but remain an amateur because I do not live from my music.

Like professional instruments I like my pro performers to look and sound good.