Okay, for once this needs to be made clear as I see that (some) people are misinformed about PCC validity and are spreading incorrect information. Unless PCC instruction for YOUR specific country says to wait for some particular milestone of the PR process, please do NOT wait post PER to get the PCC process started.
There is only a requirement of getting PCC issued within the 3 months of initial application submission, if one is submitting PCC along with the application right upfront. Post that stage there is no such 3 month condition, and a PCC can be obtained one day after the application submission and that is okay too. THIS IS SPECIFICALLY FOR FASTER PROCESSING VISA OFFICES LIKE CPC-OTTAWA, NEW DELHI ETC. that process applicants and generally issue PR within 8 to 10 months. The only thing CIC states on PCC validity other than the above is that all PCCs are considered valid by CIC for 1 year, unless a shorter validity duration is mentioned on the PCC itself by the issuing authority. Normally, most PCCs don't mention validity periods (exception can occur though) but only have a date of issue for the PCC.
In essence when a PCC is valid for 1 year from the date of issue, for faster processing VOs, there is no point in not initiating the PCC process right after Fee Charge or PER. I, in fact, initiated mine right after application submission in May 2014, and both my PCCs are issued in June 2014 itself though my PER came in August.
Police certificates Source: http://www.cic.gc.ca/english/information/applications/guides/EG7TOC.asp
Note: If you are including your police certificates with your application to the CIO, please ensure that your certificates were not issued more than three months earlier. Police certificates are generally valid for 12 months from the date they are issued. However, if a final decision is not made on your application before the expiry of your police certificate, the visa office may request that you provide a new one.