(Note: This question is in particular for low-volume self/manual assembly, i.e., without automatic pick and place machine).
Generally, stencils are used for well-aligned application of solder paste to a PCB.
Suppose you were to follow that stage with a second, similar stencil but this one for part placement, where the cutouts on this custom-made "part-stencil" are made to match the dimensions of your components.
Then one could place this part-stencil over the PCB, then use your vacuum-pickup or tweezers to quickly drop the components into these cutouts/slots/windows on the part-stencil thus making aligned placement easier and faster. Then you could lift the part-stencil, and move the PCB to reflow.
Could one make such a part-stencil approach work, or does it have any critical issues? Is it or some similar variant used?
For example, I see some issues such as the components being nudged when this part-stencil is removed, but if you play with the tolerances on the cutout slots, and set up this part-stencil slightly offset above the PCB, it might work (?)
(Above idea is inspired by a comment made by Scott under this blog post)