I'm new to Solidworks but I've free form solid modeling in KeyCreator for the last 8 years.

I'm typically designing products which are assemblies and all the parts go together. I understand the differences between assemblies and parts, but I'm confused as to where I should be working on my parts in detail. When working on a part usually you have reference data and mating parts that you turn on and off frequently. I'm trying to figure out the best workflow and best practices. Also, is this the "top down design' on the assembly side that I keep hearing about?

Thanks!