Visitor the key to working smart in Photoshop is to understand non-destructive editing. Why shouldn't you ever use the eraser tool?
Let's find out.
As a designer one of the most important skills you need to understand in Photoshop is compositing. Compositing is essentially the blending of two or more images together. The pros understand the value of non-destructive editing and use Layer Masks instead of the Eraser Tool.
Don't forget ... white conceals, black reveals. Don't ERASE! Use MASKS!
Visitor compositing is not only the foundation for A LOT of design work in Photoshop, the technique is also widely used in Motion Graphics and Film.
Think Rivendell in Lord of the Rings. That was created in 3D Modeling program like Maya and composited into the film.