Knowing which Pikmin's going to come up and controlling your line so that you'll have the right Pikmin in the near future for when you'll need them, are two different things. And sometimes I'll save my whistle if I know the next one is going to be a Purple Whistle.
Edit: I've been working on bouncing whistles a lot lately, actually. I've been doing something and haven't mentioned it yet, but I'll run at my opponent, bounce a whistle backwards to fake them out, and then bair towards them. Bair has a lot of range so it will either hit them or keep them away long enough, and we won't have much lag on our landing if you time the beginning of the bair right. And a lot of the time, opponents will drop their shield if you whistle bounce away from them, and they'll try to chase you.