If you queue as 5, then yes, it's a thing.
If you queue 5 you can do whatever you want. Because you are a 5man queue, you literally just ignore what it assigns. How is this hard to understand (not saying you, but other people I play with...)
For real though, if they could implement it into ranked. It would be fun and terrible at the same time. For double jungle and stuff.
That must have changed then. It used to say that not enough rolls are filled when that happened. I almost never play in groups so I didn't know that.
If you have less than 5, it still says you need to fill more roles. But with 5 you can do whatever you want and figure it out in champ select.