I'm creating a client content type. One of the properties I want to store for each client is their postcode.
I could create my own site column, say Client Postcode. But there's also a site column provided by SharePoint out-of-the-box, named ZIP/Postal code in the Core Contact and Calendar Columns section.
Is it better practice to always create my own site columns, or is it a good idea to try to reuse existing columns where applicable?
Create you own. As Mike said, you are then not restricted to how it is defined in the built in, but you are also not dependent on the feature the column is a part of being activated whenever you want to use your custom content type.
Naming your own columns will give you the same outcome, but the site columns are there for a "template" column if you decide not to customize it. I would say creating a custom column would be the way to go in any case, since you are not forced by the format/settings of the site column.
Under my opinion if you are creating a custom solution based on SharePoint, its always good to create your own site columns/content types and name them according to some convention (eg. Client Postcode). This would further assist you in easily in identifying your application dependencies as well as maintaining your application in case of any upgrade scenarios.
With all due respect, it has been my experience that "developers" tend to create something new when it really isn't necessary. Can you give me a technical reason for not re-using an existing Site Column? If so, why is it still there even in ShPt 2013?!? In fact, I make it a point to use existing Site Columns whenever possible. Instead of designing for the possibility of an exception, I handle the exception when it arises.
