I used to work with Pro-Engineer a long time ago and they had a great feature in assembly mode where you can create a "skeleton model" while you are in an assembly. You can do extrusions, cuts, etc. It has all the same functions as creating a part. This way when you do top down design all of the parts you created in the assembly can be linked directly to the "skeleton" model. This way any changes that are made to the model will change all of your parts accordingly. I looked into this a little to see if Solidworks can do something like this. The only thing that I could find is that I need to create sketches in the assembly to control my parts. Is this true?