Inheriting and extending OpenERP models (also known as objects, base classes) where possible, instead of creating your own, is a best practice.

1) Your new functionality can easily be added without changing existing functionality. 2) OpenERP new functionality can easily be added without requiring significant changes to your new functionality. 3) You can re-use the data structure, functions and user interfaces OpenERP has already created for the base classes. 4) You will minimize duplicate code, since you only need to write code specific to your customization. 5) You continue the flexibility of the framework in case people want to customize YOUR customization. 6) Your customization is more likely to be accepted into the product if it does not create new models.

See also

https://accounts.openerp.com/forum/Help-1/question/46

https://doc.openerp.com/trunk/server/03_module_dev_02/