I don't get much difference in effect sitting or standing using a stone holder. And I am fine just doing a few light finish passes with the stone in my palm. The "heavy" work or setting, correcting the bevel, simple scratch removal progression, I really prefer the stone holder with the choice of both hands or one instantly available. This is where the extra precision of stability comes into play.
The same for stone lapping. imho it is more accurate to have either the stone or lap stationary rather than one in each hand