I like mine rectangular (though I don't mind square on baby quilts). When making a quilt for a bed, I normally measure the mattress plus whatever amount I want for the drop (allowing a little extra for shrinkage). Then I adjust my pattern accordingly. This usually means I am adding blocks one way or the other (or both). This way my quilt fits the bed the way I like it to fit!
Actually, thinking a bit more on this - I don't mind a square quilt - so long as it fits my bed the way I want it to fit. But generally - my quilt will be rectangular.
I like the other ideas given for making a square quilt rectangular. Sometimes I am so close to the size I want (I prefer my quilt to be a little long rather than come up short) - but I would rather not have to add another entire row (and sometimes adding that row would throw off the balance of the pattern - so I would then need to add a horizontal and vertical row). I'll have to refer back to this thread for alternative ideas when I am just inches away.