Under what conditions, if any, would you explicitly tell ATC that you
are rolling, that is, actually rolling on the runway for take-off?

I hear this used occasionally, but I don't know if there are any
formal circumstances that merit it, or if it is just at pilot
discretion, or what. It is normally independent of the readback of
the take-off clearance (which precedes it), although sometimes I hear
"Rolling" without a readback of the clearance.

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