[grsnovi]

It is often a matter of trial and error. It can be very easy to create a set of very simple sketches within parts and put them together as an assembly to do simple 2D kinematic analyses. If those parts are parameter driven, making changes gets so much easier. Remember, a dimension on a sketch might be the only parameter you need. Once you start fully constraining your sketches and making certain dimensions depend on other dimensions, adjusting a complex part can be a snap!

One thing to remember (in regard to the original question) is that you can start with a large "flat" assembly (no subassemblies) and "push" parts down into logical subassemblies when and if you need to. If you push (demote) ten parts into a new subassembly, all of the constraints between those ten parts will be kept. Constraints from those parts to parts outside the group will be lost, so you will have to create them again.