Some airports now require transponder on prior to taxi anyway (pull up the atis for kbdl for example)
and furthermore I like seeing the tcas as I taxi. you will notice that the airacraft on the active runway go amber if you are taxing towards that runway. A further clue to avoid a bad day.
And last but not least, having the transponder on MIGHT allow a controll tower with ground radar to yell at a wayward aircraft with a proper callsign, rather than "aircraft approaching taxiway Alpha HALT." If I am lost I might not know what taxi way I am approaching, but my callsign I should know...
So to answer you questions, I turn em on immediately now...
Cheers
Wino