That depends on your definition of your auto number.

If it's for a primary key, then I would generate it when saving the record. It would save a read/write operation to the disk (to get and increment the id) when the user cancel the operation.

If it need to be displayed to the user while he is creating a new transaction (and it is not a primary key), then I would generate it when adding the transaction.

>In a multi user environment is it a good way to generate the auto number when the user clicks add or to generate it when the user clicks save.