There's 101 good reasons to leave the Flaps alone until 'All Clear' by the ground crew, well alluded to in the previous responses.
Our SOP requires that after "All Clear", that Flaps be set immediately prior to taxy, the intention being to avoid forgetting them during a long taxy. What does concern me, and was the first thought in my mind upon reading the opening post, is that Cold Weather ops require leaving the wing clean until just prior to takeoff, to avoid possible ice accretion in the various flap assemblies during taxy.
With "Flaps" now being a part of the 'After Start' checklist on our (B777) ECL, it concerns me that the potential exists, with winter coming, to miss them on the 'Before Takeoff' checklist if the item has been overriden on the 'After Start' checklist. Yup, I know that the checklist should be held until all open items are completed, but one more hole in the Swiss cheese..........
Regards,
Old Smokey