I like to share my experience here:
I landed on Aug. 11, and was told that my PR card should arrive in 3 months.
Then I need to attend a conference in the States last week. I know I should not travel abroad before receiving the PR card to avoid troubles. But I checked online, the risk is that the airlines won't allowed me to board. I have a (seemingly) valid work permit and visitor visa (overheard that they should become invalid after receiving PR status), and the worst scenario is that I have to cross the border via private vehicles. So I bought the flight ticket anyway, I was willing to take the risk.
On my flight back, the people at the airport counter checked my passport and work permit, and issued me boarding passes with no question. The officer at the border asked no question regarding my PR and let me in. Unbelievably smooth process!
Again, my work permit is valid until Sep 2018, and I was willing to take any RISK. I just want to share my experience in case you face the same dilemma, and don't intend to mislead anyone.