Blink182,
My personal method is -
Start-up and taxi out - Xpndr stby
Power checks - Xpndr on (Mode A)
Entering RWY - Xpndr Mode C/S
Although I believe the 'norm' is STBY for taxi and Mode C/S during power checks.
If you're at a larger airport it aids in preventing clutter on the controller's screen.