I always sit during my own personal gigs, but generally, I stand during demos...clinics can be either or.

If I was doing vocals, I'd probably stand, because it's easier to sing.

I have a bench and stand that puts the keyboard at exactly the right height when sitting, so there's no wrist issues,(my gigs are usually 3 hours) and I have a heavy duty double X-stand that I use when standing, that also maintains the proper hand position.

I prefer sitting, as I am naturally lazy.

Ian