For pocket draw, auto or revolver, I put my thumb where the hammer spur would be, if the gun had a hammer spur.
(Since my pocket guns are a 442 and/or a Kahr MK9, they don't have hammer spurs, but...)
My thumb in this position effectively prevents snagging on the lip of the pocket.
My thumb in that position when holstering an auto helps prevent the slide from being pushed out of battery.
For that matter, my thumb in that position when holstering a revolver in an IWB or OWB holster would let me feel any movement of the hammer, in the event of a trigger impingement of some sort.
IE, it's a pretty good position for the thumb when drawing or holstering.