Heat dissipation by convection for ICs placed upside down
Yes the convection effect you describe does exist even in passive cooling. Therefore some manufacturers of e.g. passively convection cooled PSU stress that their units be installed on vertical walls.
For IC sitting on a horizontal PCB, mounting them on the upper side is slightly preferred, but the majority of the heat will be led away by copper planes and can be dissipated elsewhere, e.g. via coolers mounted on top even when the IC is mounted on bottom