Hi,
The default warehouse selected for a sale order is based on the current company.
The domain which is used to select the default warehouse is,

[('company_id', '=', self.env.company.id)]

and the method is,

def _get_default_warehouse_id(self):

Hence the default warehouse will be the first warehouse shown in the tree view of warehouses under the inventory module. Here there is a drag option to arrange the warehouses. So make sure the warehouse which has to be selected by default is on the top of the tree view.
If you have any other criteria in selecting the default warehouse then try updating the domain used in the function.

Regards