I'd say with custom work it would show a great deal of lack of faith in the client which means you either need a better relationship with the client or no relationship at all.

You took the words right out of my mouth <s>. In the last 11 years, the only time I have ever been ripped off was when I had a contract. All my other work has been done on a handshake. I have also fired clients who whinged and complained about invoices because I do not want clients who do not believe that they are getting excellent value for the money they are spending.