A name like "first_result" would also make it clearer to readers that
passing that parameter has an impact on the length of the output
series (since you're injecting an extra result), and also that the
production of the first result skips calling func completely (as can
be seen in Tim's str coercion example).

So where I'd be -1 on:

    >>> list(accumulate(1, 2, 3))
    [1, 3, 6]
    >>> list(accumulate(1, 2, 3, start=0))
    [0, 1, 3, 6]
    >>> list(accumulate(1, 2, 3, start=1))
    [1, 2, 4, 7]

I'd be +1 on:

    >>> list(accumulate(1, 2, 3))
    [1, 3, 6]
    >>> list(accumulate(1, 2, 3, first_result=0))
    [0, 1, 3, 6]
    >>> list(accumulate(1, 2, 3, first_result=1))
    [1, 2, 4, 7]