Default Partner into Customer based on Sales Rep

We allocate our sales reps based on post coding rules which generates the territory and the sales rep when the lead/prospect/customer record is first saved.

What we would like to do is also default a partner into the customer record at the same time. We have the default partner stored in a custom field in each sales rep employee record, so we have access to the data. We may want to override this default partner in the customer record later on.

Question is, do we need a script to carry out this initial defaulting of the partner into the customer record once the sales rep is determined from the post code and if so which custom code function should we apply the code to, or is there another way to default this information in only when the partner field has not been initially entered?