In most of the cases before the medicals....As in my case they made verification calls before they sent me the Provisional acceptance letter.And if you will see the stages of Canadian immigration then u will find out that, this is stage 2 when your documents are being verified and then stage 3 is Interview and if stage 3 is waived, then they send you the medical forms. But at the same time i belive that till PR visa is not issued , till they can verify anything if feel any doubt.