I have almost always created sketches ( on different planes) in an assembly first before I create any parts. I find this is the easiest way to drive any changes to any parts within the assembly.
A co-worker of mine creates the main sketch(es) within a part of the assembly. I find this kind of frustrating when I have to work within his models and have to go searching for the sketches hidden in one of the parts.
Is this a good practice or not and will it cause any update problems?