You sometimes need a current representation of your file to make selections. I work a lot with Color Range for selections, and it is convenient to make a merged visible layer to use as source material for this. If you are not familiar with this technique, simply make the top layer of your psd the active layer and then use “merge visible” in the layers drop down adding the (<alt>) [<option>] key to prevent the layers from flattening. The keyboard shortcut is (<ctrl> <alt> <shift> E) [<cmd> <option> <shift> E]. This creates a new layer at the top incorporating all the layers below into a single layer, much the same as flattening the file but retaining the file layers below it.
I use this merged visible layer as the source for making the selections. If there are several adjustment layers at the top of your stack you will discover that selecting with a non pixel layer as the source will result in no pixels being selected. Making the merged layer and using it for the source will solve the problem.
Once your selection is made you can add an adjustment layer, or jump the selected pixels to a new layer for more work. With this done I drag the merged layer to the trash. This keeps you from expanding the file size for one thing, but also eliminates the loss of the ability to modify the layers under the merged layer. Keeping your pixel layers at the bottom helps you maintain flexibility.
Another good use for the merged visible layer is sharpening. I use a merged visible layer at the top of the stack for sharpening at the end of the process. This layer can easily be removed if necessary to make changes in the file, or modify the sharpening technique. Sharpening can also be adjusted based on output and sizing parameters. Adding a mask to the sharpening layer allows local sharpening, and adding layer style modifications using BlendIf can protect highlight and shadow areas from clipping with some sharpening methods.