if you put a dummy clip on an audio channel (a clip of nothing... with the volume at -inf...) in the corresponding scene, you only have to click the scene button to both change tempos AND start the click...
When we hit spontaneous sections, the click is usually shut off so the leader can feel less rigid. But often times that leader will look back and immediately want to start a song - but that song might be located 7 scenes lower

. Normally you would have to use the scene launch button AND click spacebar or play... if there is any sort of audio whatsoever, you can avoid that extra spacebar which can make things feel more fluid.
This is also convenient if you're starting a song with click only and firing loops part way into it... you can start it with one click and be immediately free to start after the count in. Works great for me.
Live+Dummy Clips+MidiController = Best metronome EVER
Ian