JOIN vs. LEFT JOIN

Hi !

I heard that it is always better to use a full JOIN rather than a LEFT JOIN, for performance reasons. Is that correct ?
But it's barely possible to use full JOINS all the time, since most of the lookup fields are not required.
Example : in the customer table, the customer type is a looup field to a customer_type table. But that is not required to post the record. So I was thinking to create a record in the customer_type table with ID = -1. And in case the customer type is NULL, to assign it the value -1. That way, it will be always possible to do a full JOIN. I was wondering if that is good practice or rather nonsense ?

Many thanks in advance,
Nico