As others have said, there are pros and cons to both Top Down and Bottom up modeling approaches. Professionally, I always just ask the customer at the beginning of a project if they have a preference for how they'd like the models constructed, or if they have any special protocols or design guidelines. It's an easy question, and quickly clears up any ambiguity!
Depending on the complexity of the model, there are a few different ways I normally go:
Simple parts or parts that will be re-used multiple times in a model:
- Model these parts in a blank part file from the origin and then use mates to constrain them in an assembly file.
Complex parts with multiple interfaces to other parts:
- Insert a blank part into an assembly and then copy the mating features surfaces into the part by doing an Offset Surface (offset = 0). Depending on the maturity of the model, I'll determine whether to allow these surfaces to remain linked through the assembly by toggling "No External References". If the mating surfaces are liable to change, it is usually safer turn on "No External References" so that the links are broken. Use these surfaces to build features from. But realize that if the parent geometry changes, you'll probably have to do a full rebuild of the model.
Complex parts with surfaces that continue from one part to another (i.e. housings):
- For this, you need to create a master model. Depending on the expected level of complexity you'll want to either:
- Just make the outer surface and cutline sketches/surfaces (very complex, high feature count), break out the surfaces into individual part files and then build features from these surfaces/sketchs
- Fully build all the parts in a single model and then break out into individual part files
As to the OPs question of "how do you convert to assembly later?", I use "Insert Part" and "Delete/Keep Body" to break out a single part file into individual part files. To do this, you open up a blank part file and select Insert > Insert Part from the top menu bar, and then select your master file. From there, you'll be able to select what information you want to carry over into the new model (i.e. bodies, surfaces, sketches, etc). Once the geometry is copied in, then select the "Delete/Keep Body" feature in the Direct Editing toolbar. Then select the body or surfaces you'd like to keep and then press ok. This way, the parts are linked and will update as the master model is updated, but you can build new features onto it with a simplified model tree.
Hope this helps!