RABC ?  got one for everyone in your party ? if not, then you'd need to do your fly-in from Atikokan side after crossing the border by car.

Cache Bay ranger station opens about mid-may ?  I assume you could self register there if its not staffed yet, but you may want to confirm that with the park HQ

you'll be going downriver and some of the portages are very close to the lip of the falls - you want to be very careful at the approach to those landings. often, there are "extensions" that should be used when the water is high (i.e. you do a longer portage to be safer). 

Park HQ and Cache Bay ranger station have a handout "map" with detailed portage locations for the falls, which aren't always accurate on some other maps - its worth getting a copy of that if you can - maybe they can mail one to you if you call and ask about Cache Bay ranger station

for those people who do use a RABC, I'm curious if you have to show it to the rangers when you get to Cache Bay or P.P. - do they ask to see it ?