Submitting the certificates...
It is better to submit the required certificates while applying. Why do you want to be responsible for the delay of cases. The rules change often.
I have submitted my Finger Print cards (with No Arrest Record Seal, from FBI) in May 2002, while i applied for the PR. I didn't furnish the Police Clearance Certificate from India, at that time. In November 2003, i again got a request to submit the Finger print cards. And, they have asked me to submit the PCC from my native country - India. It took about 25 days to get the PCC from Indian Consulate. Who knows, if i had sent it while applying itself, they wouldn't have asked for the FBI certificate for the second time, along with the PCC.