There is no hard and fast rule regarding this subject.

Things are progressing well. Don't do anything <u><em>to rock the boat</em></u>.

Let us admit that we could not <u><em>heap coals of fire on his head</em></u>.

He has accomplished a <u><em>Herculean task</em></u>.

To wrangle over an ass's shadow

It is clear that the ideas of both reformers <u><em>ran in the same groove</em></u>.

The involvement of teachers in the scheme of education proves to be a <u><em>mare's nest</em></u&gt…

To throw dust in one's eyes

A tall order