Everyone should do what they want but I just called the Canada Border Services Agency and the agent I spoke with was quite certain and clear that it must be valid the entire time you are there.  This is second time I have verified this.  Without any prompting, the agent said that you could be stopped at any time by an agent and it must be valid, and not expired.  The border agents don't give a hoot about a provincial park permit.

I'm sure people have done it, and since I have never had to show my RABC to anyone, I could have done it, but I didn't.  Overwhelming odds that you could do a lot of trips in Quetico without ever bothering because enforcement is practically zero.  But to be legally there, after entering remotely, you must be in possession of a valid - unexpired - RABC.

If someone has some actual proof that the agent I just spoke with was giving out misinformation, please post with some source information.