Some questions are more urgent than others. Some issues completely block what you're working on, while others are just asked out of curiosity. For those urgent questions, it would be nice if you could preset a bounty to be started as soon as the question is eligible for a bounty, if there are no answers yet. Any answer on tHe question will cause the bounty not to be started, so the asker has the option to just accept the answer without starting a bounty.
Am I missing some good reason to require manual action to start a bounty?