This was what I did just before Fall, after some back and forth arguments with my school's immigration specialist. She did not agree with the PER letter being a sufficient proof for approval-in-principle. In the end, I told her that all schools in Ontario should follow the same guidelines, and York University explicitly states that the PER letter for PhD stream should be enough in getting the students the eligibility for local tuition rates.
http://calendars.registrar.yorku.ca/2012-2013/financial/fee_categories/index.htm
They decided to look further into the issue and clarified further with the ministries. Lo and behold, roughly three weeks later, they sent a university-wide announcement, saying that all students who have received positive eligibility review for the PhD stream are qualified for local tuition rates, and that they should present the copy of the email to the graduate studies office so their residency eligibility can be reviewed. I believe this rule applies Ontario wide and some other schools, I think, have followed suit.
Also, don't forget that you qualify for OHIP three months after your PER date. The thing is, it would take roughly three months before you get the card in your hand anyway, so as soon as you get your PER, go to the nearest ServiceOntario, and get your eligibility for OHIP reviewed. They will reject you, for which you can then appeal (and provided that they haven't rejected your PR application, the appeal decision will be positive).
The appeal would take about 8 weeks, and the card will arrive in 6 weeks. In the decision letter, it will state that you won't be eligible until at least three months of your PER date, but most likely, by the time your card arrives, it would've been over the three months period anyway.