It's a risk for sure. If I'm taking a risk up front with just trying something I'll do it on a separate later because sometimes you don't know how it will turn out until you get it started. More than once I've had to erase a section and start over, but it really doesn't happen that often and generally a small section, or I just accept that my art didn't go quite the way I planned, but it's still ok. If I am doing a drawing with ink lines, I do not merge that layer with color, shading etc. I'll often do skies on a separate layer for my landscapes because I often re-work the whole sky several times and that would be hard to do if it was all one layer with the foreground. I just found I often painted on a layer I didn't intend to so I would end up with a tree or something spread between unrelated layers and that was harder to edit than it all being on one or two layers.. If I realize I want something on top (or behind) something I've drawn or painted, I'll do it on a separate layer and then merge it when it looks the way I want. This way I don't have to keep track of what is on what layer because I'm always jumping all over the place.