Do you create the MDPI drawable first and just scale it accordinly to the .075/1.00/1.50/2 ratio by pixels in photoshop or do you recreate each individual drawable?
I generally start big, and move to smaller.
I find that powerpoint is actually a very nice tool for creating resources for my applications. All of the graphics are vector, so they scale up and down without any quality loss.
I tend to start with the big ones if for no other reason than it is easier to work with something that looks bigger. when I move to the smaller sized ones I generally zoom in some to compensate.
Any graphic object in powerpoint will allow you to right click it and choose "Save as picture" which will output it as a png file for you. Only thing left is to drop it into draw9patch if need be.