Disclaimer: I'm no pro...but I did FF my dog by myself to some pretty great success.
It's definitely not "necessary" to stop all retrieves, but it is beneficial. To be completely honest, I didn't stop all retrieves, but I cut it down to something like 2 or 3 fun bumpers every other week.
In my understanding, cutting retrieves is beneficial because it cuts down on the opportunities for a dog to do it "wrong." Each retrieve that the pup bobbles, drops, or refuses the bumper is a chance for it to learn negative behaviors. Once you get the whole way through FF the pup should have a much better grip on how you would like it to handle the retrieves, and if it doesn't respond the way you desire, you have a tool to respond with.