Opera singers train to sing in various positions, including lying on their fronts over a table.... like so many things, it is entirely possible to perform well in different positions... but the ideal one for singing is standing erect and straight (none of this leaning hand on hip with the spine in an S shape). Having said that, I prefer to sit, because I'm a lazy cow.

When sitting to sing, try to sit as upright as possible (not always possible in the average pub chair which is stupid because a scrunched spine means a scrunched stomach so less room for beer)! I sit either with my ample ass pushed firmly back into the seat - which in some cases takes my feet off the floor which is disturbing, or on the edge of my seat, back straight, feet planted on the floor. I hate tall stools, as I have short leges, so if seated on one of those, will hop off and stand to sing, which defeats any egotistical object as I'm usually shorter standing up than when sitting on tall stools.

As with the 'book or no book' argument, follow your own preferene with an eye to local custom. In most informal singarounds, it matters not what position you are in, just so long as you make a pleasant noise and enjoy yourself.

LTS