For anyone who wants an adult response to saving daily challenges between event acts (as compared to pre-event), basic math dictates that you should plan to save as many tasks as possible UNLESS completing a task or tasks will allow you to get an extra bonus round of donuts. The same logic applies at the end of an event - if getting more event currency won't get you extra bonus rounds, you're better off saving tasks until the event is over and trying to switch to donut-earning tasks.
Again, to each their own and I apologize for the childish banter my initial response generated. I'm just trying to help the majority of players understand why the "pro tip" above is against your best interests. Sure, it may make you feel better in Act 1, but it means fewer donuts in the long run.