I wouldn't suggest duplicating the background layer. Not to create a mask like this... This is a very old technique that was popular before they had layers, before CS, and its the reason they have layers to begin with. It also can be problematic when you work with large files. I would say that its much better to learn how to work with layers, they are very efficient.
I would also not suggest flattening anything unless you had to. I have heard George DeWolfe state that you should make changes right on the image, be a man, or some such nonsense, and with all due respect to George (he can print very well) I would never do this. Certainly not for a client.
I know there are people who are going to get good results by doings things differently than I do. However, for my part, I can't imagine doing this kind of work without a tablet and brush. It takes literally half the time it would to spot things, and the ability to work with the selections via painting them is tremendous...
When I first got trained I didn't have a tablet and the trainer wouldn't come back to my office until I got one... There's a lot of things in Photoshop that can be done many ways. The program is deep. However, usually 12 out of 15 of the ways to do something are cumbersome. IMO it's best to learn the tools the pros use and make them your own. For example it took me a long time to get the curves dialog down, but I knew I had to. I use curves now 85-90% of the time. I don't ever use Levels, just took it out of the toolbox... It takes long time to learn adjustment layers well enough to understand most of what they can do. Adding and subtracting them is magic. Blending modes, well that's a whole 'nother world.