General rule of thumb that I've always followed and read from references such as experts like Ralph Kimball is that if you can find a clear correlation between referral source and the customer, then it belongs in the DimCustomer dimension. Exceptions to that rule depend on how large DimCustomer grows in records with each attribute you add.

As each customer will only likely have one referral source, it should be safe as long as there is correlation. In this case, I would be willing to bet there is a very strong relation to referral source and customers where someone will want to know how many customers came from what referral sources.

In instances where you have to pull multiple customers and their correlated attributes like referral source, then how is that any different from any other attribute to the customer like name, account date and so forth? If you don't see a reason to split out those other attributes, then you can assume referral source is also safe to leave in DimCustomer.