Today’s Question: Is it really necessary to make sure that a selection in create in Photoshop is absolutely perfect before using that selection to make a layer mask for a targeted adjustment?
Tim’s Quick Answer: Actually, my personal preference is to not worry very much at all about whether a selection is perfect before using that selection as the basis of a layer mask in Photoshop.
More Detail: In many respects you can think of selections and layer masks in Photoshop as being the exact same thing used in different contexts. Both serve as a form of stencil, identifying specific areas of an image while excluding other portions of an image. Therefore, in the context of a targeted adjustment or a composite image, you can achieve the same result regardless of whether you start with a perfect selection or instead work to refine the resulting layer mask.
The key difference here, however, is that with a layer mask you are better able to evaluate the final result by seeing the actual impact on the underlying image. When you are working to refine a selection, you aren’t actually able to see the precise effect of your work in the final image. Instead you have to use a bit of imagination, especially in the context of a selection with a feathered (soft) edge.
As a result, I prefer (and recommend) taking an approach where you create a good basic selection, but don’t worry too much about precision while creating that selection. Then, when the selection has been created, you can use that selection as the basis of a layer mask for either a targeted adjustment or a composite image. At that point you can refine the layer mask to perfection based on a preview of the actual effect in the final image. This enables you to work much more efficiently, since you can evaluate your work on the layer mask based on how your refinements impact the intended result for your photo.