In my career, in CATIA, I found two different design philosophies, in part design.

1. Design a part considering its standalone origin (planes and axis) and, after design, place a part in the assembly with geometrical constrains. In this way, it's easier to perform operations like mirror, patterns, etc.

2. Design a part in the contest of the assembly (all the parts have a common origin). In this way, part is already placed where it's necessary, and it's not mandatory to use constrains (also copy and paste is very useful) 

For large assemblies, the second is better, even if you need more operations and you have to extract external references, but you do not lose constrains.

Which one do you use or prefer?