Sure this is one way to do it. I think it would be much easier to rip them of a emulator. You know record your sprite in the right resolution and remove the unwanted pixel in a graphic editor. If you go through the assembly process what might give you from non to very much work depending on the game your working on you could also document and provide files to make it easy for other to implement there now graphics after.
And thanks for ripping graphics was happy to pick some up in the past ;P
Also YY-CHR and tilemolester are also great choices to work with ROM graphics.