This page http://book.civicrm.org/user/current/contributions/setup/ mentions
Be careful when editing core Financial Types or adding new types, because CiviCRM has useful built-in functionality that depends on the core Financial Types.
Does anyone know more details ?
More concrete question: my client uses different accounting codes for membership registration and membership renewal. Can I use a different financial type for membership registration (other than 'member dues') without losing functionality ?